Compute m_forceCompostingMode lazily.

This means code outside RenderLayerCompositor doesn't need to
know about this state and that we can get rid of the nasty
early return in updateForceCompositingMode.

Review URL: https://codereview.chromium.org/237353005

git-svn-id: svn://svn.chromium.org/blink/trunk@171670 bbb929c8-8fbe-4397-9dbb-9b2b20218538