Combine settings about compositing scrollable frames

Combine acceleratedCompositingForScrollableFramesEnabled and
compositedScrollingForFramesEnabled. They are always enabled or disabled
together.

Now they are always enabled on Android. On other platforms, they will be
automatically enabled when forceCompositingMode is true on HDPI devices.

BUG=304900

Review URL: https://codereview.chromium.org/239533002

git-svn-id: svn://svn.chromium.org/blink/trunk@171685 bbb929c8-8fbe-4397-9dbb-9b2b20218538