Don't force layout for scrollTo(0,0).

Source/core/frame/LocalDOMWindow.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 1192 matching lines @@
     ScrollBehavior scrollBehavior = ScrollBehaviorAuto;
     ScrollableArea::scrollBehaviorFromString(scrollToOptions.behavior(), scrollBehavior);
     scrollBy(x, y, scrollBehavior);
 }
 
 void LocalDOMWindow::scrollTo(double x, double y) const
 {
     if (!isCurrentlyDisplayedInFrame())
         return;
 
-    document()->updateLayoutIgnorePendingStylesheets();
+    if (x || y)

[Rick Byers, 2015/04/09 17:46:11]

can you add a brief comment saying why?  I.e. 'Force a layout only when it's
possible the specified scroll position isn't valid'

[rune, 2015/04/09 21:45:05]

Done.

+        document()->updateLayoutIgnorePendingStylesheets();
 
     if (std::isnan(x) || std::isnan(y))
         return;
 
     DoublePoint layoutPos(x * frame()->pageZoomFactor(), y * frame()->pageZoomFactor());
     scrollViewportTo(frame(), layoutPos, ScrollBehaviorAuto);
 }
 
 void LocalDOMWindow::scrollTo(const ScrollToOptions& scrollToOptions) const
 {
     if (!isCurrentlyDisplayedInFrame())
         return;
 
-    document()->updateLayoutIgnorePendingStylesheets();
-
     FrameView* view = frame()->view();
     if (!view)
         return;
 
     FrameHost* host = frame()->host();
     if (!host)
         return;
 
+    if (!scrollToOptions.hasLeft()
+        || !scrollToOptions.hasTop()
+        || scrollToOptions.left()
+        || scrollToOptions.top()) {
+        document()->updateLayoutIgnorePendingStylesheets();
+    }
+
     double scaledX = 0.0;
     double scaledY = 0.0;
 
     DoublePoint currentOffset = frame()->isMainFrame()
         ? DoublePoint(host->pinchViewport().visibleRectInDocument().location())
         : view->scrollableArea()->scrollPositionDouble();
     scaledX = currentOffset.x();
     scaledY = currentOffset.y();
 
     if (scrollToOptions.hasLeft()) {
@@ skipping 404 matching lines @@
     return m_frameObserver->frame();
 }
 
 v8::Handle<v8::Object> LocalDOMWindow::wrap(v8::Handle<v8::Object> creationContext, v8::Isolate* isolate)
 {
     ASSERT_NOT_REACHED(); // LocalDOMWindow has [Custom=ToV8].
     return v8::Handle<v8::Object>();
 }
 
 } // namespace blink