Remove PostIdleTask() from WebThreadBase.

The function is only used in a test, and no production code does not
rely on it. So it can be removed entirely. This simplifies the
implementation of each subclass of WebThreadBase.

Bug: 826203
Change-Id: I746a61b324d63fd189b8b8b6cbe496760dedad0e
Reviewed-on: https://chromium-review.googlesource.com/1092221
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565935}
9 files changed