Rename acceleratedCompositingForFixedPosition to preferCompositingToLCDText.

This adds a preferCompositingToLCDText setting, and makes it be set by the
setAcceleratedCompositingForFixedPositionEnabled() WebSettings method so
that behaviour is not changed. Next, Chromium will be changed to call
setPreferCompositingToLCDTextEnabled instead so we can remove this method
from settings.

Later, other accelerated compositing trigger settings will be merged with
preferCompositingToLCDText.

R=abarth
BUG=365851

Review URL: https://codereview.chromium.org/476273003

git-svn-id: svn://svn.chromium.org/blink/trunk@180392 bbb929c8-8fbe-4397-9dbb-9b2b20218538