Made top controls work with virtual viewport pinch-to-zoom. (Blink)

Together with the Chromium-side patch, makes top controls work properly
under the virtual viewport mode of pinch zoom. The key changes on the
Blink side are to adjust both the PinchViewport and FrameView max scroll
bounds to account for the change in the top controls offset on the
compositor. Additionally, I changed these to make the same floating point
and snapping calculations as in the compositor to prevent losing pixel
fractions.

Chromium side: https://codereview.chromium.org/641873003/

BUG=364109

Review URL: https://codereview.chromium.org/643473002

git-svn-id: svn://svn.chromium.org/blink/trunk@183639 bbb929c8-8fbe-4397-9dbb-9b2b20218538