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This CL adds the OffMainThreadWebSocket flag to enable the off-main-thread
WebSocket for WebWorkers. With the flag, WebSocket connection requests on worker
threads are not routed via the main thread. Also, this CL runs layout tests and
web-platform-tests with the flag. Failing tests will be fixed by following CLs.

Design Doc: https://docs.google.com/document/d/1ya3sfQ5YsDtHSCXn2renQjCxM4AWeYlhehw8Skppsr8/edit?usp=sharing

Bug: 825740
Change-Id: I9fe5a98632a02eb97c0d32d63917088af7f69994
Reviewed-on: https://chromium-review.googlesource.com/979835
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547726}