Fixed visible contents size calculation under scale with scrollbars.

The visible contents rects calculated in FrameView didn't account
properly for scrollbars when scaled. With the desktop pinch we need
to include the fact that scrollbars will scale along with the page.
This patch changes the visibleContentsRect method to subtract the
scrollbar thickness only after converting to CSS pixels.

Additionally, WebViewImpl's method to clamp scroll offset at a
certain scale also did not account for scaled scrollbars.

BUG=336539

Review URL: https://codereview.chromium.org/151453003

git-svn-id: svn://svn.chromium.org/blink/trunk@167342 bbb929c8-8fbe-4397-9dbb-9b2b20218538