Fix potential crashes in NetworkHandler code

chrome/browser/extensions/api/vpn_provider/vpn_provider_apitest.cc

 // Copyright 2015 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.
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/macros.h"
 #include "base/strings/string_number_conversions.h"
 #include "chrome/browser/chromeos/profiles/profile_helper.h"
@@ skipping 100 matching lines @@
   int send_packet_counter_ = 0;
   std::vector<char> ip_packet_;
 };
 
 class VpnProviderApiTest : public ExtensionApiTest,
                            public NetworkConfigurationObserver {
  public:
   VpnProviderApiTest() {}
   ~VpnProviderApiTest() override {}
 
+  void SetUpOnMainThread() override {
+    ExtensionApiTest::SetUpOnMainThread();
+    NetworkHandler::Get()->network_configuration_handler()->AddObserver(this);
+  }
+
+  void TearDownOnMainThread() override {
+    ExtensionApiTest::TearDownOnMainThread();
+    NetworkHandler::Get()->network_configuration_handler()->RemoveObserver(
+        this);
+  }
+
   void SetUpInProcessBrowserTestFixture() override {
     ExtensionApiTest::SetUpInProcessBrowserTestFixture();
     test_client_ = new TestShillThirdPartyVpnDriverClient();
     DBusThreadManager::GetSetterForTesting()->SetShillThirdPartyVpnDriverClient(
         make_scoped_ptr(test_client_));
   }
 
   void AddNetworkProfileForUser() {
     static_cast<FakeShillProfileClient*>(
         DBusThreadManager::Get()->GetShillProfileClient())
         ->AddProfile(
             kNetworkProfilePath,
             chromeos::ProfileHelper::GetUserIdHashFromProfile(profile()));
     content::RunAllPendingInMessageLoop();
   }
 
   void LoadVpnExtension() {
-    NetworkHandler::Get()->network_configuration_handler()->AddObserver(this);
     extension_ = LoadExtension(test_data_dir_.AppendASCII("vpn_provider"));
     extension_id_ = extension_->id();
     service_ = VpnServiceFactory::GetForBrowserContext(profile());
     content::RunAllPendingInMessageLoop();
   }
 
   bool RunExtensionTest(const std::string& test_name) {
     GURL url = extension_->GetResourceURL("basic.html?#" + test_name);
     return RunExtensionSubtest("vpn_provider", url.spec());
   }
@@ skipping 318 matching lines @@
   extension_service->BlacklistExtensionForTest(extension_id_);
   content::RunAllPendingInMessageLoop();
   EXPECT_FALSE(DoesConfigExist(kTestConfig));
   EXPECT_FALSE(DBusThreadManager::Get()
                    ->GetShillProfileClient()
                    ->GetTestInterface()
                    ->GetService(service_path, &profile_path, &properties));
 }
 
 }  // namespace chromeos