Fix webkit layout tests with SkDDL

SendResults can't be called on GpuMain thread or we crash. This CL
calls it from display compositor thread after we've ensured that
the appropriate GL context is current.

Change-Id: Ifa5c2cffafb5cfa55e5b21b1b93b6f1bc8d6e1e1
Reviewed-on: https://chromium-review.googlesource.com/c/1351577
Commit-Queue: Jonathan Backer <backer@chromium.org>
Reviewed-by: weiliangc <weiliangc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611395}