Blink scheduler: Move RAILModeObserver out from WebThreadScheduler

*.h for RAILModeObserver implementors can avoid including large
web_thread_scheduler.h by this CL.

This CL reduces the pre-processed size of platform/heap/handle.h from
3.07MB to 1.59MB

Bug: 242216
Change-Id: I95fef4e3e0dcafe5c4f5ee1f6ba77c4095a41ca1
Reviewed-on: https://chromium-review.googlesource.com/1149427
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577851}
20 files changed