Remove obsolete enterForceCompositing function

Nothing calls this any more after
https://codereview.chromium.org/323633002/

BUG=none

Review URL: https://codereview.chromium.org/322553006

git-svn-id: svn://svn.chromium.org/blink/trunk@175812 bbb929c8-8fbe-4397-9dbb-9b2b20218538