[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 d4a75a4a9c1f5d7099287f350142e4b631f1bd45..a8dbe12f68f7e652fa7d233289bc9cf4ed7fcccf 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -2295,6 +2295,9 @@ const char kMediaRouterEnableCloudServices[] =
 // user.
 const char kMediaRouterFirstRunFlowAcknowledged[] =
     "media_router.firstrunflow.acknowledged";
+// A list of website origins on which the user has chosen to use tab mirroring.
+const char kMediaRouterTabMirroringSources[] =
+    "media_router.tab_mirroring_sources";
 #endif
 
 // The base64-encoded representation of the public key to use to validate origin