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

chrome/test/BUILD.gn

Index: chrome/test/BUILD.gn
diff --git a/chrome/test/BUILD.gn b/chrome/test/BUILD.gn
index 42c2eb22936cf0ac1794b82d0e23077ae4caf266..cb246d3a886221b338727e4b693fcd272a9d5418 100644
--- a/chrome/test/BUILD.gn
+++ b/chrome/test/BUILD.gn
@@ -3770,6 +3770,7 @@ test("unit_tests") {
       "../browser/media/router/discovery/dial/dial_service_unittest.cc",
       "../browser/media/router/discovery/mdns/dns_sd_registry_unittest.cc",
       "../browser/media/router/discovery/media_sink_service_base_unittest.cc",
+      "../browser/media/router/event_page_request_manager_unittest.cc",
 
       # TODO(crbug.com/646627):
       # Move media_router_ui_service_factory_unittest.cc to chrome/browser/ui.