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This avoids having to copy the data between std::vector<char> and IOBuffer within the implementation of WebSocketChannel.

As a follow-up, I plan to see about eliminating a similar copy at the Mojo bindings layer in content/browser/websockets/.

Review-Url: https://codereview.chromium.org/2267233002
Cr-Commit-Position: refs/heads/master@{#422846}