Don't update main thread scroll reasons on a throttled frame.

When a frame is throttled, its compositing state cannot be queried,
so don't try to set main thread scrolling reasons.  This fixes
DCHECK failures when root layer scrolling is enabled.

BUG=490942
TEST=FrameThrottlingTest with RLS enabled
R=skobes@chromium.org

Review-Url: https://codereview.chromium.org/2662153004
Cr-Commit-Position: refs/heads/master@{#447962}

third_party/WebKit/Source/core/frame/FrameView.cpp

@@ -4889,10 +4889,13 @@ void FrameView::updateSubFrameScrollOnMainReason(
   if (!frame.isLocalFrame())
     return;
 
-  if (!toLocalFrame(frame).view()->layerForScrolling())
+  FrameView& frameView = *toLocalFrame(frame).view();
+  if (frameView.shouldThrottleRendering())
+    return;
+  if (!frameView.layerForScrolling())
     return;
 
-  reasons |= toLocalFrame(frame).view()->mainThreadScrollingReasonsPerFrame();
+  reasons |= frameView.mainThreadScrollingReasonsPerFrame();
   if (GraphicsLayer* layerForScrolling = toLocalFrame(frame)
                                              .view()
                                              ->layoutViewportScrollableArea()