Use RunLoop to wait for readback on compositor unittest

In layer unittest ReadPixel loops multiple times to wait to a readback.
This causes flake on cros valgrind bot. Instead use RunLoop to wait for
readback.

The flake is exposed if ui::Compositor uses Scheduler (CL 535733002)

BUG=403528

Review URL: https://codereview.chromium.org/587863002

Cr-Commit-Position: refs/heads/master@{#295972}
1 file changed