WebUI: Add helper FireWebUIListener method on WebUIMessageHandler.

content/public/browser/web_ui_message_handler.h

Index: content/public/browser/web_ui_message_handler.h
diff --git a/content/public/browser/web_ui_message_handler.h b/content/public/browser/web_ui_message_handler.h
index c8d3b09fcdd5fbc4bf0aa49dac6a753b336745b1..0582651ce4a716547ac96ed96a7ab273831cede8 100644
--- a/content/public/browser/web_ui_message_handler.h
+++ b/content/public/browser/web_ui_message_handler.h
@@ -10,6 +10,7 @@
 #include "base/gtest_prod_util.h"
 #include "base/logging.h"
 #include "base/strings/string16.h"
+#include "base/values.h"
 #include "content/common/content_export.h"
 #include "content/public/browser/web_ui.h"
 
@@ -17,7 +18,6 @@ class WebUIBrowserTest;
 
 namespace base {
 class ListValue;
-class Value;
 }
 
 namespace content {
@@ -91,6 +91,16 @@ class CONTENT_EXPORT WebUIMessageHandler {
   void RejectJavascriptCallback(const base::Value& callback_id,
                                 const base::Value& response);
 
+  // Helper method for notifying Javascript listeners added with
+  // cr.addWebUIListener() (defined in cr.js).
+  template <typename... Values>
+  void FireWebUIListener(const std::string& event_name,

[Dan Beam, 2017/05/18 00:08:44]

why do you have to implement this in the .h file?

[dpapad, 2017/05/18 00:41:32]

I tried moving the implementation to the .cc file, but did not succeed, throws a
linker error (I am either doing something wrong, or it is not possible), here is
the diff I tried https://pastebin.com/cKcvQZje.

Also looked for examples at
https://cs.chromium.org/search/?q=%22typename...%22&p=3&type=cs, and almost all
search results are implementing such function within the header files (with the
exceptions of some test.cc files which don't have a corresponding header file).

[Dan Beam, 2017/05/18 03:00:53]

aight, lgtm until a wizard tells us how to do this

+                         const Values&... values) {
+    // cr.webUIListenerCallback is a global JS function exposed from cr.js.
+    CallJavascriptFunction("cr.webUIListenerCallback", base::Value(event_name),
+                           values...);
+  }
+
   // Call a Javascript function by sending its name and arguments down to
   // the renderer.  This is asynchronous; there's no way to get the result
   // of the call, and should be thought of more like sending a message to