Bug 1130455 - Remove the mozasyncpanzoom attribute. r=roc

This attribute used to force APZ to be used on content processes even if the
overall APZ pref was false. However, this has a couple of problems, which cancel
each other out:
- If the pref is false, the APZ machinery is never created, and so it's
  impossible to have content processes "using" APZ.
- The reftest harness checks for the pref and ignores mozasyncpanzoom when
  evaluating the "asyncPanZoom" condition in reftest manifests.
Therefore, any reftests which were skip-if(!asyncPanZoom) would never run unless
the pref was set, and in those cases the mozasyncpanzoom attribute would not be
needed at all, as APZ is already enabled with the pref.
However, the mozasyncpanzoom attribute would cause some parts of the child
process code to behave as though APZ was enabled, which is incorrect. Removing
this attribute and relying solely on the pref corrects that.

dom/base/nsFrameLoader.cpp

@@ -2148,11 +2148,7 @@ nsFrameLoader::TryRemoteBrowser()
   nsCOMPtr<mozIApplication> containingApp = GetContainingApp();
   ScrollingBehavior scrollingBehavior = DEFAULT_SCROLLING;
 
-  if (Preferences::GetBool("layers.async-pan-zoom.enabled", false) ||
-      mOwnerContent->AttrValueIs(kNameSpaceID_None,
-                                 nsGkAtoms::mozasyncpanzoom,
-                                 nsGkAtoms::_true,
-                                 eCaseMatters)) {
+  if (Preferences::GetBool("layers.async-pan-zoom.enabled", false)) {
     scrollingBehavior = ASYNC_PAN_ZOOM;
   }
 

dom/base/nsGkAtomList.h

@@ -605,7 +605,6 @@ GK_ATOM(mouseover, "mouseover")
 GK_ATOM(mousethrough, "mousethrough")
 GK_ATOM(mouseup, "mouseup")
 GK_ATOM(mozaudiochannel, "mozaudiochannel")
-GK_ATOM(mozasyncpanzoom, "mozasyncpanzoom")
 GK_ATOM(mozfullscreenchange, "mozfullscreenchange")
 GK_ATOM(mozfullscreenerror, "mozfullscreenerror")
 GK_ATOM(mozpasspointerevents, "mozpasspointerevents")

layout/tools/reftest/reftest.js

@@ -271,7 +271,6 @@ this.OnRefTestLoad = function OnRefTestLoad(win)
     gBrowser.setAttribute("id", "browser");
     gBrowser.setAttribute("type", "content-primary");
     gBrowser.setAttribute("remote", gBrowserIsRemote ? "true" : "false");
-    gBrowser.setAttribute("mozasyncpanzoom", "true");
     // Make sure the browser element is exactly 800x1000, no matter
     // what size our window is
     gBrowser.setAttribute("style", "padding: 0px; margin: 0px; border:none; min-width: 800px; min-height: 1000px; max-width: 800px; max-height: 1000px");