[Media Router WebUI] Prevent error when route becomes null

[Media Router WebUI] Prevent error when route becomes null

Once the route-details element is stamped, getSinkForRoute_ is bound to
currentRoute_. This becomes null if the route is stopped which causes
getSinkForRoute_ to throw an innocuous but noisy error. This change adds
a check of the route before dereferencing it.

BUG=614144
NOTRY=true
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2053743002
Cr-Commit-Position: refs/heads/master@{#399281}
(cherry picked from commit 16bc9a232bd6ef00891eb061687d23f77ee29efd)

[btolsch, 2016-06-14 09:20:24]

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

[btolsch, 2016-06-14 09:20:25]

Merging to M52, no conflicts. PTAL, thanks!

[apacible, 2016-06-14 13:35:19]

rs lgtm

[btolsch, 2016-06-14 17:30:06]

The CQ bit was checked by btolsch@chromium.org

[commit-bot: I haz the power, 2016-06-14 17:30:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/2065023002/1

[commit-bot: I haz the power, 2016-06-14 17:33:25]

Committed patchset #1 (id:1)