Add testRunner.runIdleTasks() to force idle tasks to run.

components/test_runner/test_runner.h

Index: components/test_runner/test_runner.h
diff --git a/components/test_runner/test_runner.h b/components/test_runner/test_runner.h
index 065fc1f8f1896bca97ae04e00ddc83ca9c89bad8..5e31aa6a365bb38808381bc35d71fd47e5dbc7f8 100644
--- a/components/test_runner/test_runner.h
+++ b/components/test_runner/test_runner.h
@@ -594,6 +594,11 @@ class TestRunner : public WebTestRunner,
   void ResolveBeforeInstallPromptPromise(int request_id,
                                          const std::string& platform);
 
+  // Immediately run all pending idle tasks, including all pending
+  // requestIdleCallback calls.  Invoke the callback when all
+  // idle tasks are complete.
+  void RunIdleTasks(v8::Local<v8::Function> callback);
+
   // Calls setlocale(LC_ALL, ...) for a specified locale.
   // Resets between tests.
   void SetPOSIXLocale(const std::string& locale);