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This migrates all of the code in chromecast/ to use
blink::WebMessagePort instead of directly manipulating
Mojo pipes, and manually encoding messages. It preserves the
existing APIs so that downstream dependents can migrate. A
further CL will rip out the deprecated API.

BUG=803242

Change-Id: I6f3b03dd982e4093804a63f8ad6086f14c2b58b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028148
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737043}