Remove #includes of web_thread.h outside Blink.

Now, (almost) nobody uses blink::WebThread in non-Blink code. This CL
removes unused #include of web_thread.h in those files.

Note that two files related to testing were still using WebThread
indirectly. They call Platform()->MainThread()->GetTaskRunner() to
obtain a task runner. These calls were replaced with scheduler::
GetSingleThreadTaskRunnerForTesting().

Bug: 826203
Change-Id: I954eb67e99f76bd992f0bcdd73498cfdccba0726
Reviewed-on: https://chromium-review.googlesource.com/c/1255747
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Fredrik Hubinette <hubbe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596164}
9 files changed