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Cast javascript channels add a window.<foo>.postMessage(message)
method to selected RenderFrame instances to allow renderers to push
data to the embedder.

The usage looks like this:
Browser: add interface during init
Renderer: bind JsChannelBindingProvider, call Register, provide JsChannelClient
Browser: stick the pid/route/channel in a list

Embedder: AddJavascriptChannels
Browser: lookup current RenderFrame and corresponding channel, call
    JsChannelClient::CreateChannel, provide JsChannel
Renderer: create JS bindings

<script>: call function
Renderer: JsChannel::PostMessage
Browser: send message to embedder

Bug: b/141864193
Test: desktop cast_shell build + custom html
Change-Id: I1425cb115f679b876a8b9cb75c418b9d2a7a1506
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863073Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708846}