The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/236fddad6b5bcd9d00ead4c19d18e6fbd459d143

commit 236fddad6b5bcd9d00ead4c19d18e6fbd459d143
Author: imcheng <imcheng@chromium.org>
Date: Tue Mar 08 18:50:42 2016

[Media Router] Do not issue a tab mirroring sinks query if tab is not valid.

It is possible to bring up the MR UI outside the context of a valid tab.
In those cases we should omit TAB_MIRROR is one of the cast modes and
not start a sinks query for the tab mirror MediaSource.

BUG= 591855 

Review URL: https://codereview.chromium.org/1761793005

Cr-Commit-Position: refs/heads/master@{#379872}

[modify] https://crrev.com/236fddad6b5bcd9d00ead4c19d18e6fbd459d143/chrome/browser/ui/webui/media_router/media_router_ui.cc
[modify] https://crrev.com/236fddad6b5bcd9d00ead4c19d18e6fbd459d143/chrome/browser/ui/webui/media_router/media_router_ui_unittest.cc