Move MessageLoopForUI creation to BrowserTaskExecutor::Create

This is in preparation for the BrowserUIThreadScheduler where we
hope to have only one location where it's set up.

Bug: 863341, 872372
Change-Id: Ia7c07adc1d60280f2aed25f5226bfce80c6aba4b
Reviewed-on: https://chromium-review.googlesource.com/c/1461393
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632797}
13 files changed