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/

Note, this was previously landed in 183639 and reverted in 183653 due
to failing tests on Mac bots.

BUG=364109

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=183639

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

git-svn-id: svn://svn.chromium.org/blink/trunk@183713 bbb929c8-8fbe-4397-9dbb-9b2b20218538