Bug 1302071 - Part 5: Bucket PresContext invalidations by transaction ID, and only deliver them when the associated composite has completed. r=tnikkel

This patch does a few things:
* Buckets invalidations by transaction ID, and sends MozAfterPaints events for them when the associated composite completes.
* Creates a separate EventualDidPaint timer for each transaction ID we have invalidations for rather than just using one.
* Removes NotifyDidPaintForSubtree(PAINT_LAYERS), as it was only necessary for the existing bucketing mechanism.

MozReview-Commit-ID: JERMsgxhPQd

layout/base/PresShell.cpp

@@ -6288,7 +6288,9 @@ public:
   }
   ~nsAutoNotifyDidPaint()
   {
-    mShell->GetPresContext()->NotifyDidPaintForSubtree(mFlags);
+    if (mFlags & nsIPresShell::PAINT_COMPOSITE) {
+      mShell->GetPresContext()->NotifyDidPaintForSubtree();
+    }
   }
 
 private: