[Cast Harmony] Remove Cast Setup from context menu.

Cast Setup in Chrome is deprecated and is being removed in M72.  This removes
the Media Router context menu item to access Cast Setup.

This patch also restricts navigation in the chrome://cast WebUI to Cast Offers, which is not being deprecated.  Tested by manually navigating to various chrome://cast URLs.

Screenshot: https://drive.google.com/open?id=1XRpTGtqiQ458eLdJ0oY72OObmU85XRfQ

Bug: 895580
Change-Id: I7c94f00cebe80246c11d17e163f5e92ed1ecbdcc
Reviewed-on: https://chromium-review.googlesource.com/c/1308085
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604662}
7 files changed