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This patch adds the following:

- GetState(), a new MediaRouteProvider API to allow the Media Router to
  query the internal state of MRPs
- GetProviderState(), a new MediaRouter API to expose this to chrome/
- Implementation of GetState() in the Cast MRP
- A call to GetState(CAST) from the Media Router Internals WebUI,
  which renders the results as JSON.

The initial use case is to allow Cast developers to access the Cast
session ID from a running session.  However, this can be extended in
the future for feedback reports and debugging.

This also adds some basic styling to the internals page for readability.
(Much more could be done here.)

Bug: 1046078
Change-Id: I20d1c2123486c70e0f5ec24243ea1c4ec016dc52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2084675
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748313}