FrameView's size should be calculated based on contentWidth+scrollbarWidth

In virtual viewport mode, the main FrameView's size is calculated based on
the content width of the page. On platforms where scrollbars take up layout
space (non-overlay scrollbars), this should include the scrollbar width as
well as the content width since the PinchViewport encompasses both when
fully zoomed out and the PinchViewport is bounded by the outer viewport.

BUG=148816

Review URL: https://codereview.chromium.org/686293003

git-svn-id: svn://svn.chromium.org/blink/trunk@184682 bbb929c8-8fbe-4397-9dbb-9b2b20218538