[Media Router] Force DEFAULT cast mode when starting presentations from content.

chrome/browser/resources/media_router/elements/media_router_container/media_router_container.js

Index: chrome/browser/resources/media_router/elements/media_router_container/media_router_container.js
diff --git a/chrome/browser/resources/media_router/elements/media_router_container/media_router_container.js b/chrome/browser/resources/media_router/elements/media_router_container/media_router_container.js
index ff9296814a7eeddcf8752044bf520113b037fe76..4010f1b73444fa92c1dd9bb9af47b7dbf10f6aab 100644
--- a/chrome/browser/resources/media_router/elements/media_router_container/media_router_container.js
+++ b/chrome/browser/resources/media_router/elements/media_router_container/media_router_container.js
@@ -530,6 +530,11 @@ Polymer({
     if (!this.castModeList.length)
       return;
 
+    // If there is a forced mode make sure it is shown.
+    if (this.findForcedCastMode_()) {
+      this.rebuildSinksToShow_();
+    }
+
     // If we are currently showing auto mode, then nothing needs to be done.
     // Otherwise, if the cast mode currently shown no longer exists (regardless
     // of whether it was selected by user), then switch back to auto cast mode.
@@ -611,13 +616,18 @@ Polymer({
   },
 
   /**
-   * If |allSinks| supports only a single cast mode, returns that cast mode.
-   * Otherwise, returns AUTO_MODE. Only called if |userHasSelectedCastMode_| is
-   * |false|.
+   * If there is a forced cast mode, returns that cast mode.  If |allSinks|
+   * supports only a single cast mode, returns that cast mode.  Otherwise,
+   * returns AUTO_MODE. Only called if |userHasSelectedCastMode_| is |false|.
+   *
    * @return {!media_router.CastMode} The single cast mode supported by
    *                                  |allSinks|, or AUTO_MODE.
    */
   computeCastMode_: function() {
+    /** @const */ var forcedMode = this.findForcedCastMode_();
+    if (forcedMode)
+      return forcedMode;
+
     var allCastModes = this.allSinks.reduce(function(castModesSoFar, sink) {
       return castModesSoFar | sink.castModes;
     }, 0);
@@ -1149,6 +1159,17 @@ Polymer({
   },
 
   /**
+   * Helper function to return a forced CastMode, if any.
+   *
+   * @return {media_router.CastMode|undefined} CastMode object with
+   *     isForced = true, or undefined if not found.
+   */
+  findForcedCastMode_: function() {
+    return this.castModeList &&
+        this.castModeList.find(element => element.isForced);
+  },
+
+  /**
    * @param {?Element} element Element to compute padding for.
    * @return {!Array<number>} Array containing the element's bottom padding
    *     value and the element's top padding value, in that order.