Bug 947753 / Bug 1122526 - Don't claim to support off-main-thread animations when the nearest widget is not using OMT compositing. r=birtles

This moves the test for whether off-main-thread compositor is enabled
*earlier* in CanPerformOnCompositorThread, since
CanAnimatePropertyOnCompositor is called only from
CanPerformOnCompositorThread.  This change means we're using a more
accurate test for whether we actually have off-main-thread compositing
than the pref, since in some cases we won't use off-main-thread
compositing for certain widgets (e.g., transparent widgets on Windows)
even when the pref is enabled.

layout/style/AnimationCommon.cpp

@@ -511,15 +511,6 @@ AnimationPlayerCollection::CanAnimatePropertyOnCompositor(
   CanAnimateFlags aFlags)
 {
   bool shouldLog = nsLayoutUtils::IsAnimationLoggingEnabled();
-  if (!gfxPlatform::OffMainThreadCompositingEnabled()) {
-    if (shouldLog) {
-      nsCString message;
-      message.AppendLiteral("Performance warning: Compositor disabled");
-      LogAsyncAnimationFailure(message);
-    }
-    return false;
-  }
-
   nsIFrame* frame = nsLayoutUtils::GetStyleFrame(aElement);
   if (IsGeometricProperty(aProperty)) {
     if (shouldLog) {
@@ -587,6 +578,19 @@ AnimationPlayerCollection::CanPerformOnCompositorThread(
     return false;
   }
 
+  nsIWidget* widget = frame->GetNearestWidget();
+  if (!widget ||
+      widget->GetLayerManager()->GetBackendType() !=
+        layers::LayersBackend::LAYERS_CLIENT) {
+    // No widget (huh?), or a widget not using off-main-thread compositor.
+    if (nsLayoutUtils::IsAnimationLoggingEnabled()) {
+      nsCString message;
+      message.AppendLiteral("Performance warning: Compositor disabled");
+      LogAsyncAnimationFailure(message);
+    }
+    return false;
+  }
+
   if (mElementProperty != nsGkAtoms::transitionsProperty &&
       mElementProperty != nsGkAtoms::animationsProperty) {
     if (nsLayoutUtils::IsAnimationLoggingEnabled()) {