Bug 969779 - Use SpecialPowers to enable preference setting in oop. r=seth

1. Use |SpecialPowers.pushPrefEnv| to enable preference setting in oop.
2. Set the time of redrawing to be the time right after the image is discarded.
3. Modify the assertion to make the comparison non-trivial.
4. Remove <img>.
5. Move drawCanvas() on the image loadComplete.

image/test/mochitest/mochitest.ini

@@ -59,7 +59,6 @@ support-files =
 
 [test_ImageContentLoaded.html]
 [test_bug399925.html]
-skip-if = e10s || buildapp == 'b2g' #Bug 969779 - should set preference via SpecialPowers.pushPrefEnv()
 # [test_bug435296.html]
 # disabled - See bug 578591
 [test_bug466586.html]

image/test/mochitest/test_bug399925.html

@@ -13,7 +13,6 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=399925
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=399925">Mozilla Bug 399925</a>
 <p id="display"></p>
 <div id="content" style="display: none">
-<img src="bug399925.gif" id="gif" />
 <canvas id="canvas" width="100" height="100"> </canvas>  
 </div>
 <pre id="test">
@@ -21,56 +20,61 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=399925
 
 /** Test for Bug 399925. **/
 var pngResults = new Array();
-var oldTimeoutPref;
-var oldDiscardPref;
 SimpleTest.waitForExplicitFinish();
-window.onload = runTest;
 
-function runTest()
-{
-  // Get the old discard timeout
-  oldTimeoutPref = getImagePref(DISCARD_TIMEOUT_PREF);
-
-  // We're testing discarding here, so we should make sure it's flipped on
-  oldDiscardPref = getImagePref(DISCARD_ENABLED_PREF);
-
-  // Enable Discarding
-  setImagePref(DISCARD_ENABLED_PREF, true);
-
-  // Sets the discard timer to 500ms so we don't have to wait so long. The
-  // actual time is nondeterministic, but is bounded by 2 * timer = 1 second.
-  setImagePref(DISCARD_TIMEOUT_PREF, 1000);
+window.onload = function() {
+  // 1. Enable Discarding
+  // 2. Sets the discard timer to 500ms so we don't have to wait so long. The
+  //    actual time is nondeterministic, but is bounded by 2 * timer = 1 second.
+  SpecialPowers.pushPrefEnv({
+    'set':[['image.mem.discardable',true],
+           ['image.mem.min_discard_timeout_ms',1000]]
+  }, runTest);
+}
 
+function runTest() {
   // Create the image _after_ setting the discard timer pref
   var image = new Image();
   image.setAttribute("id", "gif");
   image.src = "bug399925.gif";
   document.getElementById("content").appendChild(image);
 
-  // draw the canvas once
-  drawCanvas();
-
-  // Set the timeout to draw it after discard
-  setTimeout('drawCanvas(); allDone();', 3000);
-}
-
-function drawCanvas()
-{
-    var canvas = document.getElementById('canvas');
-    var context = canvas.getContext('2d');
-    var gif = document.getElementById('gif');
-
-    context.drawImage(gif, 0, 0);
-    ok(true, "we got through the drawImage call without an exception being thrown");
-    pngResults.push(canvas.toDataURL);
-}
-
-function allDone()
-{
+  // 1. Draw the canvas once on loadComplete
+  // 2. Redraw the canvas and compare the results right on discard
+  addCallbacks(image, drawCanvas, function() {
+    drawCanvas();
     is(pngResults[0], pngResults[1], "got different rendered results");
-    setImagePref(DISCARD_TIMEOUT_PREF, oldTimeoutPref);
-    setImagePref(DISCARD_ENABLED_PREF, oldDiscardPref);
     SimpleTest.finish();
+  });
+}
+
+function addCallbacks(anImage, loadCompleteCallback, discardCallback) {
+  var observer = new ImageDecoderObserverStub();
+  observer.discard = function () {
+    imgLoadingContent.removeObserver(scriptedObserver);
+    discardCallback();
+  }
+  observer.loadComplete = loadCompleteCallback;
+  observer = SpecialPowers.wrapCallbackObject(observer);
+
+  var scriptedObserver = SpecialPowers.Cc["@mozilla.org/image/tools;1"]
+                           .getService(SpecialPowers.Ci.imgITools)
+                           .createScriptedObserver(observer);
+
+  var imgLoadingContent =
+    SpecialPowers.wrap(anImage)
+                 .QueryInterface(SpecialPowers.Ci.nsIImageLoadingContent);
+  imgLoadingContent.addObserver(scriptedObserver);
+}
+
+function drawCanvas() {
+  var canvas = document.getElementById('canvas');
+  var context = canvas.getContext('2d');
+  var gif = document.getElementById('gif');
+
+  context.drawImage(gif, 0, 0);
+  ok(true, "we got through the drawImage call without an exception being thrown");
+  pngResults.push(canvas.toDataURL());
 }
 
 </script>