Remove m_forcedCompositingMode from RenderLayerCompositor.

We don't need this bit anymore. m_compositing serves it's purpose,
except in CompositingRequirementsUpdater::updateRecursive, where
we can just query the setting directly as needed.

RenderLayerCompositor::updateIfNeeded would unecessarily
do all it's work and then disabled compositing if hasAcceleratedCompositing
returned false. Instead, we can early return. As far as I
know this only affects SVGImages at the moment, but there
should be no change in behavior.

Review URL: https://codereview.chromium.org/242383002

git-svn-id: svn://svn.chromium.org/blink/trunk@172066 bbb929c8-8fbe-4397-9dbb-9b2b20218538