Remove WebScheduler::LoadingTaskRunner()

In ResourceLoader.cpp, WebScheduler::LoadingTaskRunner() is used to
make sure all the posted tasks are executed even after the frame is
gone, but now it is assured posted tasks are executed correctly before
the task runner is destroyed, LoadingTaskRunner() is no longer needed.

This is part of efforts to replace thread-local task runners with other
appropriate per-frame task runners in the renderer.

Bug: 801476
Change-Id: If0a0ee06694cf6a5d7d4447824d4e236cb8dd85d
Reviewed-on: https://chromium-review.googlesource.com/863844
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529229}
15 files changed