Merge AcceleratedCompositingForOverflowScroll into PreferCompositingToLCDText.

This removes the AcceleratedCompositingForOverflowScroll setting and
changes code that checked for it to instead check for the
PreferCompositingToLCDText setting.

R=abarth@chromium.org, vollick@chromium.org, abarth
BUG=365851

Review URL: https://codereview.chromium.org/486493002

git-svn-id: svn://svn.chromium.org/blink/trunk@180492 bbb929c8-8fbe-4397-9dbb-9b2b20218538