Rename WebThread to Thread in the entire code base.

The class is no longer exposed to //content and others. This CL rewrites
all the occurrences of the old name in the entire code base, and removes
the temoprary aliases.

Bug: 826203
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I98ef06a8cd87e168216257eb76ac8bad1c662ceb
Reviewed-on: https://chromium-review.googlesource.com/c/1261455
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598703}
76 files changed