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Add a callback argument to WebSocket Send() for string and ArrayBuffer.
The callback will be called when the send completes, if it happens
asynchronously and the callback is non-null.

The callback is not used yet. It will be used by the WebSocketStream
implementation.

The methods return SENT_SYNCHRONOUSLY if they were able to send the data
over mojo synchronously, as in that case the callback will not be
called.

The Blob Send method hasn't been changed, as it won't be used by
WebSocketStream.

See design doc for WebSocketStream at
https://docs.google.com/document/d/1XuxEshh5VYBYm1qRVKordTamCOsR-uGQBCYFcHXP4L0/edit

BUG=983030

Change-Id: Ia7f3147a49e8bc55f8ead63e611a395124118959
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1718445
Commit-Queue: Adam Rice <ricea@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682209}