Revert 161821 "Remove 'FAST_MOBILE_SCROLLING'"

The removal forced main-thread scrolling for a large number of websites.
Reverting until a permanent fix can be found.

BUG=327175
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/103533011

git-svn-id: svn://svn.chromium.org/blink/trunk@164361 bbb929c8-8fbe-4397-9dbb-9b2b20218538