Move WebThreadScheduler::{Add,Remove}TaskObserver() to ThreadScheduler.

Those functions are not used by any Chromium (non-Blink) code, thus they
should not be exposed via Web interfaces.

This patch moves the functions to ThreadScheduler, which is the right
home for this kind of functions. As WebThreadScheduler and
ThreadScheduler's interfaces are implemented by the same concrete
class, the amount of the code changes are kept minimum.

Bug: 826203
Change-Id: Ifcbad156a7e3a2788294681ca544afdcba7faabd
Reviewed-on: https://chromium-review.googlesource.com/1065555
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@{#559886}
6 files changed