s/scheduleLayerUpdate/setNeedsCompositingUpdate in setWebLayer.

This was added in http://src.chromium.org/viewvc/blink?view=revision&revision=80584.
It didn't have tests, so I'm not sure how to test this.
But plugins always have a RenderLayer, so there should be no
need to schedule a layer update anymore.

Review URL: https://codereview.chromium.org/318773003

git-svn-id: svn://svn.chromium.org/blink/trunk@175597 bbb929c8-8fbe-4397-9dbb-9b2b20218538