Simplify ExtensionInstallChecker into a single-use class

extensions/test/extension_test_notification_observer.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef EXTENSIONS_TEST_EXTENSION_TEST_NOTIFICATION_OBSERVER_H_
 #define EXTENSIONS_TEST_EXTENSION_TEST_NOTIFICATION_OBSERVER_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 28 matching lines @@
   void WaitForExtensionLoad();
 
   // Waits for an extension load error. Returns true if the error really
   // happened.
   bool WaitForExtensionLoadError();
 
   // Wait for the specified extension to crash. Returns true if it really
   // crashed.
   bool WaitForExtensionCrash(const std::string& extension_id);
 
-  // Wait for the crx installer to be done. Returns true if it really is done.
+  // Wait for the crx installer to be done. Returns true if it has finished
+  // successfully.
   bool WaitForCrxInstallerDone();
 
   // Watch for the given event type from the given source.
   // After calling this method, call Wait() to ensure that RunMessageLoop() is
   // called appropriately and cleanup is performed.
   void Watch(int type, const content::NotificationSource& source);
 
   // After registering one or more event types with Watch(), call
   // this method to run the message loop and perform cleanup.
   void Wait();
@@ skipping 74 matching lines @@
 
   // The closure to quit the currently-running message loop.
   base::Closure quit_closure_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionTestNotificationObserver);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_TEST_EXTENSION_TEST_NOTIFICATION_OBSERVER_H_