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

chrome/browser/ui/webui/media_router/media_cast_mode.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_UI_WEBUI_MEDIA_ROUTER_MEDIA_CAST_MODE_H_
 #define CHROME_BROWSER_UI_WEBUI_MEDIA_ROUTER_MEDIA_CAST_MODE_H_
 
 #include <set>
 #include <string>
 
 namespace media_router {
 
 // A cast mode represents one way that the current WebContents (i.e., tab) may
 // be presented to a media sink.  These must be declared in the priority order
 // returned by GetPreferredCastMode.
 enum MediaCastMode {
-  // The default presentation for the WebContents.  Only available when the
-  // document has provided a default presentation URL.
+  // A presentation URL provided by the WebContents via the Presentation API.
+  // This can be set by the default presentation URL (for top level browsing
+  // contexts) or presentation URLs passed with a PresentationRequest (for top
+  // level and nested browsing contexts).
+  // TODO(mfoltz): More accurately named PRESENTATION
   DEFAULT = 0x1,
   // Capture the rendered WebContents and stream it to a media sink.  Always
   // available.
   TAB_MIRROR = 0x2,
   // Capture the entire desktop and stream it to a media sink.  Always
   // available.
   DESKTOP_MIRROR = 0x4,
 };
 
 using CastModeSet = std::set<MediaCastMode>;
 
 // Returns a localized description string for |mode| and |host|
 // (e.g. google.com).
 std::string MediaCastModeToDescription(MediaCastMode mode,
                                        const std::string& host);
 
 // Returns true if |cast_mode_num| is a valid MediaCastMode, false otherwise.
 bool IsValidCastModeNum(int cast_mode_num);
 
 }  // namespace media_router
 
 #endif  // CHROME_BROWSER_UI_WEBUI_MEDIA_ROUTER_MEDIA_CAST_MODE_H_