[CAF MR] Implement CafBaseMediaRouteProvider

This CL adds CafBaseMediaRouteProvider, which corresponds to
BaseMediaRouteProvider. The new provider is the base class
containing the common logic for CAF MediaRouteProviders.

Bug: 711860
Change-Id: I0783854b990730ebf6a5bbf5bcaa10ba090ee7c1
Reviewed-on: https://chromium-review.googlesource.com/1168138
Commit-Queue: Zhiqiang Zhang <zqzhang@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582687}
6 files changed