Deflake inspector layer tests

- add quietUntilDone flag that suppresses inspector test output till test is
    done, so output does not interfere with layout & co;
- only install LayerTreeChanged event listener after we actually schedule tree
    update to avoid stray events;
- drop testRunner.display() and use requestAnimationFrame to avoid multiple
   tree changes being coalesced.
- speed up layer-compositing-reasons.html by parallelizing all queries.

Review URL: https://codereview.chromium.org/149273003

git-svn-id: svn://svn.chromium.org/blink/trunk@166038 bbb929c8-8fbe-4397-9dbb-9b2b20218538