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This allows using //base/task/post_task.h for posting tasks to a
WebThread by specifying a WebThread::ID as a task trait.

Related patch for BrowserThreads: https://crrev.com/c/1181364

Bug: 867421, 878356
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Ic4a539eadc1fda199376c42afa44ba95e39e7041
Reviewed-on: https://chromium-review.googlesource.com/1188283
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587089}