Make PluginResponseInterceptorURLLoaderThrottle work on UI thread

We are in the process of migrating throttles to the UI thread for
navigation. For now, throttles will be able to run on both UI and IO
threads, this migrates PluginResponseInterceptorURLLoaderThrottle to be
able to work on UI.

Bug: 824845
Change-Id: I4785910f42497be9eaa2adb84af8ff3327e72be0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674644
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672275}
5 files changed