Add WebWidget::compositeAndReadbackAsync().

This method goes through WebLayerTreeView to do a composite and
readback for layout tests with pixel output, but does so in an
asynchronous manner.

When compositing is not enabled, it just returns false.

R=abarth, enne
BUG=251973

Review URL: https://codereview.chromium.org/257003005

git-svn-id: svn://svn.chromium.org/blink/trunk@172824 bbb929c8-8fbe-4397-9dbb-9b2b20218538