Remove unused external message loop support from base::Thread

The callsites were removed in crrev.com/618761.

Bug: 826465
Change-Id: If1b36e9b95b83183653bf5c927d8bdd8dbee7959
Reviewed-on: https://chromium-review.googlesource.com/c/1478893
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634300}
3 files changed