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
Fix inaccurate comment.

Clearing the StyleResolver does not cause the whole document to be
recalced. The FIXME is still valid I think, just for a different reason.

Review URL: https://codereview.chromium.org/80873004

git-svn-id: svn://svn.chromium.org/blink/trunk@162562 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Do not arbitrarily snap scroll animation to 60Hz

Doing so artificially limits the frame rate and introduces
aliasing.

A complete solution would use a global animation timestamp
for the current frame, but that is left for a followup patch
to this one.

BUG=320955

Review URL: https://codereview.chromium.org/79093004

git-svn-id: svn://svn.chromium.org/blink/trunk@162563 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Make "Not available" tooltip in Heap profiler less confusing

BUG=319578

Review URL: https://codereview.chromium.org/83753003

git-svn-id: svn://svn.chromium.org/blink/trunk@162564 bbb929c8-8fbe-4397-9dbb-9b2b20218538