Rename WebThreadImplForWorkerScheduler to WorkerThread.

This is part of refactoring on WebThread classes.

Bug: 826203
Change-Id: I9c2d86e7009aaa9a1053d662d114d8f00536d795
Reviewed-on: https://chromium-review.googlesource.com/c/1295761
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601952}
7 files changed