Disable fractional scrolling on main thread

Fractional scroll offset on main thread is causing regression for
position-fixed element. To prevent the regression goes into M40,
this CL disables the main fractional scrolling by rounding scroll
offset of scroll event to be integer. After M40 branches, we will
revert this CL and figure out how to correctly fix the regression
for position-fixed element.

BUG=422854, 414283

Review URL: https://codereview.chromium.org/687003003

git-svn-id: svn://svn.chromium.org/blink/trunk@184551 bbb929c8-8fbe-4397-9dbb-9b2b20218538