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/about_flags.cc b/chrome/browser/about_flags.cc
index defc7a1f..949db24 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2305,6 +2305,7 @@
     SINGLE_VALUE_TYPE(chromeos::switches::kEnableFirewallHolePunching)
   },
 #endif  // defined(OS_CHROMEOS)
+#if defined(ENABLE_MEDIA_ROUTER)
   {
     "enable-media-router",
     IDS_FLAGS_ENABLE_MEDIA_ROUTER_NAME,
@@ -2312,6 +2313,7 @@
     kOsAll,
     SINGLE_VALUE_TYPE(switches::kEnableMediaRouter)
   },
+#endif  // defined(ENABLE_MEDIA_ROUTER)
 // Since kEnableLauncherSearchProviderApi is not available when app list is
 // disabled, flag guard enable-launcher-search-provider-api.
 #if defined(ENABLE_APP_LIST)