s/scheduleLayerUpdate/setNeedsCompositingUpdate in setRemoteWebLayer.

Now that frames always get a RenderLayer, we don't need to
actually do a layer update anymore.

Not sure how to test this. The original patch that added it
https://codereview.chromium.org/104433003 didn't have any
new tests.

Review URL: https://codereview.chromium.org/318863002

git-svn-id: svn://svn.chromium.org/blink/trunk@175542 bbb929c8-8fbe-4397-9dbb-9b2b20218538