[Media Router] Allow casting new media to sink with existing route.

chrome/browser/resources/media_router/elements/route_details/route_details.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <dom-module id="route-details">
   <link rel="import" type="css" href="../../media_router_common.css">
   <link rel="import" type="css" href="route_details.css">
   <template>
     <div id="route-information" hidden$="[[!isCustomControllerHidden_]]">
       <span>[[activityStatus_]]</span>
     </div>
     <extensionview id="custom-controller"
         hidden$="[[isCustomControllerHidden_]]">
     </extensionview>
     <div id="route-action-buttons" class="layout">
       <paper-button flat class="route-button button"
           id="start-casting-to-route-button"
-          hidden$="[[!route.canJoin]]"

[amp, 2016/05/23 21:42:35]

It would be nice to update this logic to make it hidden if the current sourceUrn
(of the page which the MR UI has been opened on) matches the sourceUrn of the
existing route.

That would require passing around more info though.  What sets the route.canJoin
property?  Maybe whatever does that can update the logic to check for the new
scenarios.

[btolsch, 2016/05/23 22:41:10]

The UI sets route.canJoin by matching a list of joinable route IDs from the
extension to the list of routes.

The route ID is indeed in a normal URN format that contains the source URN. 
However, the URN prefix and format is currently only known to the component
extension.  Without an extension change, we'd have to add route URN parsing
somewhere either in MR to pass the information up to the UI or in the UI itself.

Since this should be a relatively temporary change, I don't feel strongly either
way.  WDYT?

[mark a. foltz, 2016/05/23 23:53:38]

Does this remove the entire logic about when to show the "CAST" button for
individual routes?  Fascinating.  Assuming that there's no other use for
route.canJoin, we could remove all the plumbing in the MR and MR UX that
determines which routes are joinable from which tabs.

Always showing the "CAST" button means there must be a default cast mode for
every page and every sink, otherwise clicking it would be undefined.  I think
the only time this might be a problem is with a DIAL device that only supports
e.g. YouTube and not mirroring. 

[btolsch, 2016/05/24 20:20:18]

route.canJoin would still be necessary, but in any case I was too quick to
remove the button hidden state.  It turns out we actually don't even get the
DIAL sinks in the allSinks, even when we get the route. So casting YouTube to a
DIAL TV and then opening the MR dialog in a new tab will show the route but the
sink isn't actually there to even check the cast modes.

I added logic to hide the button again when there's no compatible cast modes for
the route's sink, which includes if the user selects a cast mode manually first.
 I also snuck in a small fix for the header text that relates to the same DIAL
condition.

           on-tap="startCastingToRoute_">
         <span>[[i18n('startCastingButtonText')]]</span>
       </paper-button>
       <paper-button flat class="route-button button"
           id="close-route-button"
           on-tap="closeRoute_">
         <span>[[i18n('stopCastingButtonText')]]</span>
       </paper-button>
     <div>
   </template>
 <script src="route_details.js"></script>
 </dom-module>