Bug 899810 - Ensure that mFrameMetrics is updated properly in NotifyLayersUpdated. r=bbondy

In some cases the mScrollOffset was getting updated on the mFrameMetrics
before the IsDefault() check, so the IsDefault() check would return false
when it should have returned true. Calling IsDefault() on the
mFrameMetrics before writing stuff into it fixes this.

gfx/layers/ipc/AsyncPanZoomController.cpp

@@ -1169,6 +1169,7 @@ void AsyncPanZoomController::NotifyLayersUpdated(const FrameMetrics& aLayerMetri
 
   mLastContentPaintMetrics = aLayerMetrics;
 
+  bool isDefault = mFrameMetrics.IsDefault();
   mFrameMetrics.mMayHaveTouchListeners = aLayerMetrics.mMayHaveTouchListeners;
 
   // TODO: Once a mechanism for calling UpdateScrollOffset() when content does
@@ -1207,7 +1208,7 @@ void AsyncPanZoomController::NotifyLayersUpdated(const FrameMetrics& aLayerMetri
     needContentRepaint |= (previousResolution != newResolution);
   }
 
-  if (aIsFirstPaint || mFrameMetrics.IsDefault()) {
+  if (aIsFirstPaint || isDefault) {
     mPaintThrottler.ClearHistory();
     mPaintThrottler.SetMaxDurations(gNumPaintDurationSamples);
 
@@ -1217,7 +1218,7 @@ void AsyncPanZoomController::NotifyLayersUpdated(const FrameMetrics& aLayerMetri
     // XXX If this is the very first time we're getting a layers update we need to
     // trigger another repaint, or the B2G browser shows stale content. This needs
     // to be investigated and fixed.
-    needContentRepaint |= (mFrameMetrics.IsDefault() && !aLayerMetrics.IsDefault());
+    needContentRepaint |= (isDefault && !aLayerMetrics.IsDefault());
 
     mFrameMetrics = aLayerMetrics;
     mState = NOTHING;