[Media Router] Make per-hostname cast mode selections persist

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index 0c464505c638aa0e28959aa0f434f64e9915c789..2ed9155c4d2b38af81b2b6efcf7895870ce904f9 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -2284,6 +2284,10 @@ const char kMediaRouterEnableCloudServices[] =
 // user.
 const char kMediaRouterFirstRunFlowAcknowledged[] =
     "media_router.firstrunflow.acknowledged";
+// A dictionary mapping website hostnames to user's preference for the cast mode
+// to use on that website.
+const char kMediaRouterCastModeSelections[] =
+    "media_router.cast_mode_selections";
 #endif
 
 // The base64-encoded representation of the public key to use to validate origin