WebUI: Add JavaScript lifecycle-control to WebUIMessageHandler.

content/public/browser/web_ui.h

Index: content/public/browser/web_ui.h
diff --git a/content/public/browser/web_ui.h b/content/public/browser/web_ui.h
index 663afd1fc45d1c25c287bc63f8e3ab4db1cab345..8d45e28b8bdc64ae842bfa6d46d9979b0fab6e37 100644
--- a/content/public/browser/web_ui.h
+++ b/content/public/browser/web_ui.h
@@ -91,10 +91,18 @@ class CONTENT_EXPORT WebUI {
                                    const std::string& message,
                                    const base::ListValue& args) = 0;
 
+  // Returns true if this WebUI can currently call JavaScript.
+  virtual bool CanCallJavascript() = 0;
+
+  // Calling these functions directly is discouraged. It's generally preferred
+  // to call WebUIMessageHandler::CallJavascriptFunction, as that has
+  // lifecycle controls to prevent calling JavaScript before the page is ready.
+  //
   // 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
   // the page.
+  //
   // All function names in WebUI must consist of only ASCII characters.
   // There are variants for calls with more arguments.
   virtual void CallJavascriptFunction(const std::string& function_name) = 0;