Don't do a style recalc when updating the composited layer of a plugin.

Since the renderer for the plugin always gets either a NormalLayer
or a ForcedLayer, it will have a RenderLayer whether it's composited
or not, so all we have to do is update it's self painting layer bit.

This is a followup to https://codereview.chromium.org/328553004.

Review URL: https://codereview.chromium.org/322193002

git-svn-id: svn://svn.chromium.org/blink/trunk@175942 bbb929c8-8fbe-4397-9dbb-9b2b20218538