Rollup of bug 819000: Preload about:blank. r=cjones,jlebar a=cjones

Includes the following patches

Bug 819000: Bail out of PresShell::Paint if not active. r=roc
Bug 819000: Make sure preloaded docshells are reset to active. r=jlebar
Bug 819000: Preload about:blank. r=cjones a=cjones

b2g/app/b2g.js

@@ -577,7 +577,7 @@ pref("hal.processPriorityManager.gonk.backgroundNice", 10);
 // (hiding latency).
 pref("dom.ipc.processPrelaunch.enabled", true);
 // Wait this long before pre-launching a new subprocess.
-pref("dom.ipc.processPrelaunch.delayMs", 1000);
+pref("dom.ipc.processPrelaunch.delayMs", 5000);
 #endif
 
 // Ignore the "dialog=1" feature in window.open.

dom/browser-element/BrowserElementChild.js

@@ -70,6 +70,11 @@ BrowserElementChild.prototype = {
 
   _init: function() {
     debug("Starting up.");
+
+    // NB: this must happen before we process any messages from
+    // mozbrowser API clients.
+    docShell.isActive = true;
+
     sendAsyncMsg("hello");
 
     // Set the docshell's name according to our <iframe>'s name attribute.

dom/ipc/ContentChild.cpp

@@ -1151,8 +1151,15 @@ ContentChild::RecvAppInfo(const nsCString& version, const nsCString& buildID)
 {
     mAppInfo.version.Assign(version);
     mAppInfo.buildID.Assign(buildID);
-
-    PreloadSlowThings();
+    // If we're part of the mozbrowser machinery, go ahead and start
+    // preloading things.  We can only do this for mozbrowser because
+    // PreloadSlowThings() may set the docshell of the first TabChild
+    // inactive, and we can only safely restore it to active from
+    // BrowserElementChild.js.
+    if ((mIsForApp || mIsForBrowser) &&
+        Preferences::GetBool("dom.ipc.processPrelaunch.enabled", false)) {
+        PreloadSlowThings();
+    }
     return true;
 }
 

dom/ipc/preload.js

@@ -69,4 +69,11 @@
   Cc["@mozilla.org/thread-manager;1"].getService(Ci["nsIThreadManager"]);
   Cc["@mozilla.org/toolkit/app-startup;1"].getService(Ci["nsIAppStartup"]);
   Cc["@mozilla.org/uriloader;1"].getService(Ci["nsIURILoader"]);
+
+  docShell.isActive = false;
+  docShell.QueryInterface(Ci.nsIWebNavigation)
+          .loadURI("about:blank",
+                   Ci.nsIWebNavigation.LOAD_FLAGS_NONE,
+                   null, null, null);
+
 })();

layout/base/nsPresShell.cpp

@@ -5246,6 +5246,10 @@ PresShell::Paint(nsIView*           aViewToPaint,
   NS_ASSERTION(!mIsDestroying, "painting a destroyed PresShell");
   NS_ASSERTION(aViewToPaint, "null view");
 
+  if (!mIsActive) {
+    return;
+  }
+
   nsAutoNotifyDidPaint notifyDidPaint(aWillSendDidPaint);
 
   nsPresContext* presContext = GetPresContext();