Add BlimpContents[,Observer] and BlimpNavigationController

blimp/client/core/public/blimp_contents_observer.h

Index: blimp/client/core/public/blimp_contents_observer.h
diff --git a/blimp/client/core/public/blimp_contents_observer.h b/blimp/client/core/public/blimp_contents_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..bd3b86a96c6660c0add1615f9156b1b7b4a83404
--- /dev/null
+++ b/blimp/client/core/public/blimp_contents_observer.h
@@ -0,0 +1,33 @@
+// 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.
+
+#ifndef BLIMP_CLIENT_CORE_PUBLIC_BLIMP_CONTENTS_OBSERVER_H_
+#define BLIMP_CLIENT_CORE_PUBLIC_BLIMP_CONTENTS_OBSERVER_H_
+
+#include "base/macros.h"
+#include "url/gurl.h"
+
+namespace blimp {
+namespace client {
+
+// An observer API implemented by classes which are interested in various events
+// related to BlimpContents.
+class BlimpContentsObserver {
+ public:
+  virtual ~BlimpContentsObserver() = default;
+
+  // Invoked when the URL of the BlimpContents is changes.
+  virtual void OnURLUpdated(const GURL& url) {}
+
+ protected:
+  BlimpContentsObserver() {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(BlimpContentsObserver);
+};
+
+}  // namespace client
+}  // namespace blimp
+
+#endif  // BLIMP_CLIENT_CORE_PUBLIC_BLIMP_CONTENTS_OBSERVER_H_