DOM: Reduce the size of document.h

This CL reduces the estimated pre-processed size of document.h from
4.36MB to 4.16MB.

* GetScheduler():
This overrides |FrameOrWorkerScheduler* ExecutionContext::
GetScheduler()|. Document::GetScheduler() returned FrameScheduler,
which is a subclass of FrameOrWorkerScheduler, but no one needs
FrameScheduler.

Bug: 242216
Change-Id: Ic998973a3dc9b934b3ea2abef1b9e35b9e9f022c
Reviewed-on: https://chromium-review.googlesource.com/1125742
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572692}
6 files changed