Move DidBeginMainFrame after the impl-thread commit.

DidBeginMainFrame can cause mutations to layer state, and
the commit needs to be complete before doing so.

Bug: 939342

Change-Id: I078a78879394f999cebe05a2804ac05617a31cbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1584498
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654699}