Bug 1540492. Fix some bugs in test_discardAnimatedImage.html. r=aosmond

Bug 1503653 added webp images to this test but didn't update everywhere necessary in the test file.

This doesn't fix any intermittents with this test as far as I know.

Differential Revision: https://phabricator.services.mozilla.com/D25527

--HG--
extra : moz-landing-system : lando

image/test/mochitest/test_discardAnimatedImage.html

@@ -114,8 +114,10 @@ function checkIfFinished() {
 
   if ((gNumFrameUpdates[0] >= kNumFrameUpdatesToExpect) &&
       (gNumFrameUpdates[1] >= kNumFrameUpdatesToExpect) &&
+      (gNumFrameUpdates[2] >= kNumFrameUpdatesToExpect) &&
       (gNumSnapShotChanges[0] >= kNumFrameUpdatesToExpect) &&
-      (gNumSnapShotChanges[1] >= kNumFrameUpdatesToExpect)) {
+      (gNumSnapShotChanges[1] >= kNumFrameUpdatesToExpect) &&
+      (gNumSnapShotChanges[2] >= kNumFrameUpdatesToExpect)) {
     ok(true, "got expected frame updates");
     gFinished = true;
     SimpleTest.finish();
@@ -129,7 +131,7 @@ function addCallbacks(anImage, arrayIndex) {
   observer.discard = function () {
     gNumDiscards++;
     ok(true, "got image discard");
-    if (arrayIndex >= 2) {
+    if (arrayIndex >= 3) {
       // The last two images are finite, so we don't expect any frame updates,
       // this image is done the test, so remove the observer.
       if (!gRemovedObserver[arrayIndex]) {