RenderLayerCompositor::setNeedsCompositingUpdate shouldn't enableCompositingModeIfNeeded

We should just be setting dirty bits. By the way, marquee is crazy.

Review URL: https://codereview.chromium.org/330653003

git-svn-id: svn://svn.chromium.org/blink/trunk@176582 bbb929c8-8fbe-4397-9dbb-9b2b20218538