Add flag to run NavigationURLLoaderImpl on UI thread

This also fixes URLLoaderInterceptor to work with this change.

Bug: 976990
Change-Id: Icd72956170dee911e51e43063951d44cc7634617
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1671067
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672159}
6 files changed