[headless] Ensure devtooled tests call Disable/RemoveObserver.

headless/lib/embedder_mojo_browsertest.cc

 // 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.
 
 #include <memory>
 #include "base/optional.h"
 #include "base/path_service.h"
 #include "base/strings/string_piece.h"
 #include "base/strings/stringprintf.h"
 #include "base/threading/thread_restrictions.h"
@@ skipping 216 matching lines @@
   void RunMojoTest() override {
     devtools_client_->GetNetwork()->AddObserver(this);
     devtools_client_->GetNetwork()->Enable();
   }
 
   GURL GetInitialUrl() const override {
     return embedded_test_server()->GetURL("/page_one.html");
   }
 
   void ReturnTestResult(const std::string& result) override {
-    FinishAsynchronousTest();
+    DisableClientAndFinishAsynchronousTest();
     FAIL() << "The HTTP page should not have been served and we should not have"
               " recieved a mojo callback!";
   }
 
   void OnLoadingFailed(const network::LoadingFailedParams& params) override {
     // The navigation should fail since HTTP requests are blackholed.
     EXPECT_EQ(params.GetErrorText(), "net::ERR_FILE_NOT_FOUND");
+    DisableClientAndFinishAsynchronousTest();
+  }
+
+  void DisableClientAndFinishAsynchronousTest() {
+    devtools_client_->GetNetwork()->Disable();
+    devtools_client_->GetNetwork()->RemoveObserver(this);
     FinishAsynchronousTest();
   }
 };
 
 DEVTOOLS_CLIENT_TEST_F(HttpDisabledByDefaultWhenMojoBindingsUsed);
 
 }  // namespace headless