Bug 799956 - Mark BasicLayerManagers that don't retain content as supporting component alpha layers. r=roc

gfx/layers/basic/BasicLayers.h

@@ -93,7 +93,7 @@ public:
   virtual void EndTransaction(DrawThebesLayerCallback aCallback,
                               void* aCallbackData,
                               EndTransactionFlags aFlags = END_DEFAULT);
-  virtual bool AreComponentAlphaLayersEnabled() { return HasShadowManager(); }
+  virtual bool AreComponentAlphaLayersEnabled() { return HasShadowManager() || !IsWidgetLayerManager(); }
 
   void AbortTransaction();