Update WebFrame::VisibleContentRect for RLS.

TESTED=RenderViewImplTest.PreferredSizeZoomed passes with RLS

Bug: 785088
Change-Id: Ibce3e168a7b777b7ee4a51593177d6cbc162ec87
Reviewed-on: https://chromium-review.googlesource.com/886837Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532048}

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

@@ -615,7 +615,7 @@ bool WebLocalFrameImpl::HasVisibleContent() const {
 
 WebRect WebLocalFrameImpl::VisibleContentRect() const {
   if (LocalFrameView* view = GetFrameView())
-    return view->VisibleContentRect();
+    return view->LayoutViewportScrollableArea()->VisibleContentRect();
   return WebRect();
 }