[Extensions] Apply WARN_UNUSED_RESULT and final keyword to ExtensionFunctions.

extensions/browser/extension_function.h

Index: extensions/browser/extension_function.h
diff --git a/extensions/browser/extension_function.h b/extensions/browser/extension_function.h
index 2e367d33be8597b8c88442b1ea5aedbc966f1806..30906299ffffc4863c83b9fa4108bcb147708ee4 100644
--- a/extensions/browser/extension_function.h
+++ b/extensions/browser/extension_function.h
@@ -321,18 +321,26 @@ class ExtensionFunction
 
   // ResponseActions.
   //
+  // These are exclusively used as return values from Run(). Call Respond(...)
+  // to respond at any other time - but as described below, only after Run()
+  // has already executed, and only if it returned RespondLater().
+  //
   // Respond to the extension immediately with |result|.
-  ResponseAction RespondNow(ResponseValue result);
+  ResponseAction RespondNow(ResponseValue result) WARN_UNUSED_RESULT;
   // Don't respond now, but promise to call Respond(...) later.
-  ResponseAction RespondLater();
+  ResponseAction RespondLater() WARN_UNUSED_RESULT;
 
   // This is the return value of the EXTENSION_FUNCTION_VALIDATE macro, which
   // needs to work from Run(), RunAsync(), and RunSync(). The former of those
   // has a different return type (ResponseAction) than the latter two (bool).
-  static ResponseAction ValidationFailure(ExtensionFunction* function);
+  static ResponseAction ValidationFailure(ExtensionFunction* function)
+      WARN_UNUSED_RESULT;
 
-  // If RespondLater() was used, functions must at some point call Respond()
-  // with |result| as their result.
+  // If RespondLater() was returned from Run(), functions must at some point
+  // call Respond() with |result| as their result.
+  //
+  // More specifically: call this iff Run() has already executed, it returned
+  // RespondLater(), and Respond(...) hasn't already been called.
   void Respond(ResponseValue result);
 
   virtual ~ExtensionFunction();
@@ -588,7 +596,10 @@ class AsyncExtensionFunction : public UIThreadExtensionFunction {
   static bool ValidationFailure(AsyncExtensionFunction* function);
 
  private:
-  ResponseAction Run() override;
+  // If you're hitting a compile error here due to "final" - great! You're
+  // doing the right thing, you just need to extend UIThreadExtensionFunction
+  // instead of AsyncExtensionFunction.
+  ResponseAction Run() final;
 };
 
 // A SyncExtensionFunction is an ExtensionFunction that runs synchronously
@@ -615,7 +626,10 @@ class SyncExtensionFunction : public UIThreadExtensionFunction {
   static bool ValidationFailure(SyncExtensionFunction* function);
 
  private:
-  ResponseAction Run() override;
+  // If you're hitting a compile error here due to "final" - great! You're
+  // doing the right thing, you just need to extend UIThreadExtensionFunction
+  // instead of SyncExtensionFunction.
+  ResponseAction Run() final;
 };
 
 class SyncIOThreadExtensionFunction : public IOThreadExtensionFunction {
@@ -636,7 +650,10 @@ class SyncIOThreadExtensionFunction : public IOThreadExtensionFunction {
   static bool ValidationFailure(SyncIOThreadExtensionFunction* function);
 
  private:
-  ResponseAction Run() override;
+  // If you're hitting a compile error here due to "final" - great! You're
+  // doing the right thing, you just need to extend IOThreadExtensionFunction
+  // instead of SyncIOExtensionFunction.
+  ResponseAction Run() final;
 };
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_FUNCTION_H_