[Media Router] Add UMA metrics for Media Router Dialog WebUI performance.

chrome/browser/ui/webui/media_router/media_router_webui_message_handler.cc

Index: chrome/browser/ui/webui/media_router/media_router_webui_message_handler.cc
diff --git a/chrome/browser/ui/webui/media_router/media_router_webui_message_handler.cc b/chrome/browser/ui/webui/media_router/media_router_webui_message_handler.cc
index ce1da635092c325752fc17ed8006473a08bc6f76..a591adc1074035138443892010bf3e94268acc7a 100644
--- a/chrome/browser/ui/webui/media_router/media_router_webui_message_handler.cc
+++ b/chrome/browser/ui/webui/media_router/media_router_webui_message_handler.cc
@@ -29,6 +29,7 @@ const char kActOnIssue[] = "actOnIssue";
 const char kCloseRoute[] = "closeRoute";
 const char kCloseDialog[] = "closeDialog";
 const char kReportSinkCount[] = "reportSinkCount";
+const char kOnInitialDataReceived[] = "onInitialDataReceived";
 
 // JS function names.
 const char kSetInitialData[] = "media_router.ui.setInitialData";
@@ -256,11 +257,16 @@ void MediaRouterWebUIMessageHandler::RegisterMessages() {
       kReportSinkCount,
       base::Bind(&MediaRouterWebUIMessageHandler::OnReportSinkCount,
                  base::Unretained(this)));
+  web_ui()->RegisterMessageCallback(
+      kOnInitialDataReceived,
+      base::Bind(&MediaRouterWebUIMessageHandler::OnInitialDataReceived,
+                 base::Unretained(this)));
 }
 
 void MediaRouterWebUIMessageHandler::OnRequestInitialData(
     const base::ListValue* args) {
   DVLOG(1) << "OnRequestInitialData";
+  media_router_ui_->OnUIInitiallyLoaded();

[mark a. foltz, 2015/12/09 22:20:13]

When is this called in the initialization flow?  It seems like we are capturing
some script execution time as well.  Does this happen after the top level
Polymer elements have been created for the default view?

[apacible, 2015/12/09 23:19:55]

This is called ASAP when the WebUI finishes loading.

The flow looks like:
1. User action to open dialog.
2. Dialog created (not shown yet).
3. WebUI page loads ('load' event fired).
4. 'load' event triggers initialize().
5. initialize() immediately calls requestInitialData()
6. requestInitialData() immediately calls OnRequestInitialData() here.
7. OnRequestInitialData() immediately calls OnUIInitiallyLoaded(). <- log here

I previously added some logging to look at initialization flow (not included in
the CL since it was just for sanity checks) and the page loading includes the
creation of the Polymer elements.

   base::DictionaryValue initial_data;
 
   // "No Cast devices found?" Chromecast help center page.
@@ -395,6 +401,12 @@ void MediaRouterWebUIMessageHandler::OnReportSinkCount(
   UMA_HISTOGRAM_COUNTS_100("MediaRouter.Ui.Device.Count", sink_count);
 }
 
+void MediaRouterWebUIMessageHandler::OnInitialDataReceived(
+    const base::ListValue* args) {
+  DVLOG(1) << "OnInitialDataReceived";
+  media_router_ui_->OnUIInitialDataReceived();
+}
+
 bool MediaRouterWebUIMessageHandler::ActOnIssueType(
     const IssueAction::Type& action_type,
     const base::DictionaryValue* args) {