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It was needed because "it's hard to handle IPC errors synchronously",
but we don't have that problem now with mojo IPC. It is not only
redundant but also harmful because it breaks a WebSocketClient contract:
OnFinishOpeningHandshake must precede OnAddChannelResponse.

Bug: 721400
Change-Id: Idd475140d4aef1d49f001855ff80fcf8d5f99cda
Reviewed-on: https://chromium-review.googlesource.com/1068784Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561392}