Remove const qualifier from WebThread::Scheduler().

This function returns a mutable pointer, thus it does not make sense to
have this function const-qualified.

Bug: 826203
Change-Id: I7ea049c9a86b79db7bab5e26bf16b0499d71a08d
Reviewed-on: https://chromium-review.googlesource.com/1247643
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594626}
7 files changed