Remove an early return from RenderLayerCompositor::updateIfNeeded.

I'm not sure what this was originally there for, but all this early return does now
is avoid a handful of branches for non-composited subframes.

Review URL: https://codereview.chromium.org/298073002

git-svn-id: svn://svn.chromium.org/blink/trunk@174877 bbb929c8-8fbe-4397-9dbb-9b2b20218538