[Media Router] Assign each route a current cast mode if possible

[Media Router] Assign each route a current cast mode if possible

This change gives each media route in the WebUI a current cast mode
value if its media source corresponds to one of the currently available
cast modes. This is to prevent showing the 'cast' button that allows
replacing the route when the new route would effectively be a copy of
the original route.

BUG=614144
NOTRY=true
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2040883002
Cr-Commit-Position: refs/heads/master@{#398687}
(cherry picked from commit a47d5b0506c1183d501767d17ffec02ca419ea76)

[btolsch, 2016-06-14 00:42:42]

btolsch@chromium.org changed reviewers:
+ apacible@chromium.org

[btolsch, 2016-06-14 00:42:42]

Merge to M52, had to undo CallJavascriptFunctionUnsafe function rename because
it hasn't been merged to M52. PTAL, thanks!

[apacible, 2016-06-14 04:15:15]

rs lgtm

[btolsch, 2016-06-14 07:23:08]

The CQ bit was checked by btolsch@chromium.org

[commit-bot: I haz the power, 2016-06-14 07:23:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/2068593002/1

[commit-bot: I haz the power, 2016-06-14 07:26:58]

Committed patchset #1 (id:1)