Remove the switch from RenderLayerCompositor::setNeedsCompositingUpdate

We now drive keep track of what kind of update to do using std::max on the
updateType. The last remaining holdout was
CompositingUpdateAfterCanvasContextChange, which now understands how to do a
partial update.

Review URL: https://codereview.chromium.org/265813004

git-svn-id: svn://svn.chromium.org/blink/trunk@173110 bbb929c8-8fbe-4397-9dbb-9b2b20218538