Merge acceleratedCompositingForFixedRootBackground into preferCompositingToLCDText

This removes the acceleratedCompositingForFixedRootBackground setting
and makes the WebSetting setter a no-op. Any code that checked for
this setting is replaced with a check for
preferCompositingToLCDTextEnabled instead.

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

Review URL: https://codereview.chromium.org/479893002

git-svn-id: svn://svn.chromium.org/blink/trunk@180431 bbb929c8-8fbe-4397-9dbb-9b2b20218538