Remove synchronous compositeAndReadback() from WebLayerTreeView.

Depends on: https://codereview.chromium.org/264713014/

R=abarth
BUG=251936

Review URL: https://codereview.chromium.org/276753002

git-svn-id: svn://svn.chromium.org/blink/trunk@173804 bbb929c8-8fbe-4397-9dbb-9b2b20218538