Headless: Target.disposeBrowserContext() should close context

The https://crrev.com/555640 regressed headless implementation of
Target.disposeBrowserContext() method and omitted the actual context
closing.

This patch brings back the missing method.

R=dgozman, alexclarke
TBR=alexclarke

Change-Id: I28677381be9fbb86daa5445978e9c0ce4bf388f7
Reviewed-on: https://chromium-review.googlesource.com/1050983
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557087}

headless/lib/browser/protocol/target_handler.cc

@@ -98,6 +98,7 @@ Response TargetHandler::DisposeBrowserContext(const std::string& context_id) {
     // close them too.
     web_contents = context->GetAllWebContents();
   }
+  context->Close();
   return Response::OK();
 }
 

headless/lib/headless_devtools_client_browsertest.cc

@@ -513,6 +513,17 @@ class TargetDomainCreateAndDeleteBrowserContextTest
 
   void OnDisposeBrowserContextResult(
       std::unique_ptr<target::DisposeBrowserContextResult> result) {
+    devtools_client_->GetTarget()->GetExperimental()->GetBrowserContexts(
+        target::GetBrowserContextsParams::Builder().Build(),
+        base::BindOnce(&TargetDomainCreateAndDeleteBrowserContextTest::
+                           OnGetBrowserContexts,
+                       base::Unretained(this)));
+  }
+
+  void OnGetBrowserContexts(
+      std::unique_ptr<target::GetBrowserContextsResult> result) {
+    const std::vector<std::string>* contexts = result->GetBrowserContextIds();
+    EXPECT_EQ(0u, contexts->size());
     FinishAsynchronousTest();
   }