[DevTools] Set correct overlay size for scaled page.

Overlay was sized to visible size, but it is not scaled together with the page.
Therefore, the correct size is equal to viewport size.

BUG=508136

Review URL: https://codereview.chromium.org/1327503002

git-svn-id: svn://svn.chromium.org/blink/trunk@201573 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/web/PageOverlay.cpp

@@ -90,7 +90,7 @@ void PageOverlay::update()
         page->frameHost().visualViewport().containerLayer()->addChild(m_layer.get());
     }
 
-    FloatSize size = page->frameHost().visualViewport().visibleSize();
+    FloatSize size = page->frameHost().visualViewport().size();
     if (size != m_layer->size())
         m_layer->setSize(size);