[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 6a7249dd3af0fbc0969acda7f36eadd82e8923cd..98d1b3e179b548de87cc951022721c64dc58678e 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -2285,6 +2285,10 @@ const char kMediaRouterEnableCloudServices[] =
 // user.
 const char kMediaRouterFirstRunFlowAcknowledged[] =
     "media_router.firstrunflow.acknowledged";
+// A dictionary mapping website hostnames to whether the user has chosen to use
+// tab mirroring on that website.
+const char kMediaRouterTabMirroringSelected[] =
+    "media_router.tab_mirroring_selected";
 #endif
 
 // The base64-encoded representation of the public key to use to validate origin