[Media Router] Factor extension-related logic out of MediaRouterMojoImpl

chrome/browser/media/router/BUILD.gn

Index: chrome/browser/media/router/BUILD.gn
diff --git a/chrome/browser/media/router/BUILD.gn b/chrome/browser/media/router/BUILD.gn
index 0855d4f90a5360694f048e92c0fab01c7deb3161..7218231160a9f27d0114f45c4be428f9898677d7 100644
--- a/chrome/browser/media/router/BUILD.gn
+++ b/chrome/browser/media/router/BUILD.gn
@@ -71,6 +71,11 @@ static_library("router") {
       "//mojo/public/cpp/bindings",
     ]
     sources += [
+      "event_page_request_manager.cc",
+      "event_page_request_manager.h",
+      "event_page_request_manager_factory.cc",
+      "event_page_request_manager_factory.h",
+
       # TODO(crbug.com/646627): Move media_router_ui_service* to
       # chrome/browser/ui/.
       "media_router_ui_service.cc",