Make HeadlessWebContentsSecurityTest disable observer before completion.

headless/lib/headless_web_contents_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 <string>
 #include <vector>
 
 #include "content/public/test/browser_test.h"
 #include "headless/public/devtools/domains/page.h"
@@ skipping 71 matching lines @@
   void RunDevTooledTest() override {
     devtools_client_->GetSecurity()->GetExperimental()->AddObserver(this);
     devtools_client_->GetSecurity()->GetExperimental()->Enable(
         security::EnableParams::Builder().Build());
   }
 
   void OnSecurityStateChanged(
       const security::SecurityStateChangedParams& params) override {
     EXPECT_EQ(security::SecurityState::NEUTRAL, params.GetSecurityState());
 
+    devtools_client_->GetSecurity()->GetExperimental()->Disable(
+        security::DisableParams::Builder().Build());
+    devtools_client_->GetSecurity()->GetExperimental()->RemoveObserver(this);
     FinishAsynchronousTest();
   }
 };
 
 HEADLESS_ASYNC_DEVTOOLED_TEST_F(HeadlessWebContentsSecurityTest);
 
 }  // namespace headless