Miscellaneous changes to finish Media Router upstreaming.
* Fix a bug which would cause the Mojo layer to disassociate itself
from the extension after the extension is suspended.
* Upstreame some missing WebUI glue code.
* Add a flag --enable-media-router-ui.
* Fix a minor bug in the Media Router mojo bindings JS.
* Partial patch of in-flight CL #
1161113003
BUG=461815
R=mfoltz@chromium.org,apacible@chromium.org
Review URL: https://codereview.chromium.org/
1173193003
Cr-Commit-Position: refs/heads/master@{#334503}