Make <canvas> always have a RenderLayer

In preparation for making HTMLCanvasElement stop using scheduleLayerUpdate
we should make it always have a RenderLayer. This removes the need to
create/destroy the RenderLayer for the <canvas> for reasons that are
not style related.

Review URL: https://codereview.chromium.org/264923006

git-svn-id: svn://svn.chromium.org/blink/trunk@173145 bbb929c8-8fbe-4397-9dbb-9b2b20218538