Add declarations for --enable-media-router.
Add URL constant for Media Router WebUI.

BUG=461815,464208

Review URL: https://codereview.chromium.org/1069443002

Cr-Commit-Position: refs/heads/master@{#324525}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 76a06f9..20d38e1 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -772,6 +772,9 @@
               "//chrome")
     }
   }
+  if (enable_media_router) {
+    defines += [ "ENABLE_MEDIA_ROUTER=1" ]
+  }
 }
 
 if (is_android) {