Migrate TabControlFeature from 0.5 to 0.6

blimp/client/public/contents/blimp_contents_observer.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 "blimp/client/public/contents/blimp_contents.h"
+#include "blimp/client/public/contents/blimp_contents_observer.h"
+
+namespace blimp {
+namespace client {
+
+BlimpContentsObserver::BlimpContentsObserver(BlimpContents* blimp_contents)
+    : contents_(blimp_contents) {
+  blimp_contents->AddObserver(this);
+}
+
+BlimpContentsObserver::~BlimpContentsObserver() {

[David Trainor- moved to gerrit, 2016/08/06 04:32:58]

Can we have two unit tests for this?

1. When this dies BlimpContents should drop the reference to it.
2. When BlimpContents dies, this should get a call to OnContentsDestroyed (which
has a valid BlimpContents), and after that BlimpContents should be null.

[Menglin, 2016/08/09 00:31:18]

Done.

+  if (contents_)
+    contents_->RemoveObserver(this);
+}
+
+void BlimpContentsObserver::OnContentsDestroyed() {
+  DCHECK(contents_);
+  ClearBlimpContents();
+}
+
+void BlimpContentsObserver::ClearBlimpContents() {
+  contents_ = nullptr;
+}
+
+}  // namespace client
+}  // namespace blimp