Clean up WebViewImpl to only allow compositing setup code to run when compositing.

Previously, a Page inside of the WebViewImpl would think that
compositing was possible, even in cases when there is no WebLayerTreeView available.
Only while setting it up would the code in WebViewImpl undo
that, which is wrong.

If there is no WebLayerTreeView, then compositing is not
possible. In that case, tell the page that compositing is
not supported, so it doesn't try to set it up.

BUG=

Review URL: https://codereview.chromium.org/408443003

git-svn-id: svn://svn.chromium.org/blink/trunk@178589 bbb929c8-8fbe-4397-9dbb-9b2b20218538