Remove the forceCompositingMode setting.

Nothing sets it to false anymore. This also means we can
remove all the machinery around setting this via
enterForceCompositingMode. Unfortunately, WebPopupMenuImpl
still needs enterForceCompositingMode. That's crbug.com/378029.

BUG=363772,378029

Review URL: https://codereview.chromium.org/295383008

git-svn-id: svn://svn.chromium.org/blink/trunk@175027 bbb929c8-8fbe-4397-9dbb-9b2b20218538