Make HeadlessWebContentsSecurityTest disable observer before completion.

headless/lib/headless_web_contents_browsertest.cc

Index: headless/lib/headless_web_contents_browsertest.cc
diff --git a/headless/lib/headless_web_contents_browsertest.cc b/headless/lib/headless_web_contents_browsertest.cc
index 18b4295c2b94e42e8db687ebd4652e6a5750af99..9f58ec731c52434e520dc07670d5389dfc35c0c0 100644
--- a/headless/lib/headless_web_contents_browsertest.cc
+++ b/headless/lib/headless_web_contents_browsertest.cc
@@ -89,6 +89,9 @@ class HeadlessWebContentsSecurityTest
       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();
   }
 };