WebSocket: Remove unnecessary off-the-main-thread WebSocket flag

Off-the-main-thread WebSocket was enabled by default in M68, so this flag is no
longer necessary.

Bug: 825740
Change-Id: I87615e7c7687f92252914221b077ea5cadd7ddec
Reviewed-on: https://chromium-review.googlesource.com/c/1286095
Reviewed-by: Adam Rice <ricea@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600321}
3 files changed