Add a setting to do frame scrolls through the root layer.

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 1559 matching lines @@
 }
 
 bool RenderLayer::hitTest(const HitTestRequest& request, const HitTestLocation& hitTestLocation, HitTestResult& result)
 {
     ASSERT(isSelfPaintingLayer() || hasSelfPaintingLayerDescendant());
 
     // RenderView should make sure to update layout before entering hit testing
     ASSERT(!renderer()->frame()->view()->layoutPending());
     ASSERT(!renderer()->document().renderView()->needsLayout());
 
-    LayoutRect hitTestArea = renderer()->view()->documentRect();
-    if (!request.ignoreClipping())
-        hitTestArea.intersect(frameVisibleRect(renderer()));
+    // Start with frameVisibleRect to ensure we include the scrollbars.
+    LayoutRect hitTestArea = frameVisibleRect(renderer());
+    if (request.ignoreClipping())
+        hitTestArea.unite(renderer()->view()->documentRect());
 
     RenderLayer* insideLayer = hitTestLayer(this, 0, request, result, hitTestArea, hitTestLocation, false);
     if (!insideLayer) {
         // We didn't hit any layer. If we are the root layer and the mouse is -- or just was -- down,
         // return ourselves. We do this so mouse events continue getting delivered after a drag has
         // exited the WebView, and so hit testing over a scrollbar hits the content document.
         // In addtion, it is possible for the mouse to stay in the document but there is no element.
         // At that time, the events of the mouse should be fired.
         LayoutPoint hitPoint = hitTestLocation.point();
         if (!request.isChildFrameHitTest() && ((request.active() || request.release()) || (request.move() && hitTestArea.contains(hitPoint.x(), hitPoint.y()))) && isRootLayer()) {
@@ skipping 1277 matching lines @@
     }
 }
 
 void showLayerTree(const blink::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif