DevTools: disable "force compositing" setting unless both modes are supported

This plumbs supported modes from WebPreferences to DevTools front-end and
only enables "force compositing" checkbox if both accelerated and non-accelerated
modes are actually spported.
Drive-by: clean-up a bunch of unused forward class declarations in WebDevTools*.h

BUG=306480

Review URL: https://codereview.chromium.org/27330002

git-svn-id: svn://svn.chromium.org/blink/trunk@162561 bbb929c8-8fbe-4397-9dbb-9b2b20218538