Add TabManager class, relocate Tab to common/.

blimp/engine/common/tab.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/engine/common/tab.h"
+
+#include <utility>
+
+#include "blimp/engine/common/page_load_tracker.h"
+#include "content/public/browser/web_contents.h"
+
+namespace blimp {
+namespace engine {
+
+Tab::Tab(int id, std::unique_ptr<content::WebContents> web_contents)
+    : web_contents_(std::move(web_contents)), id_(id) {}
+
+Tab::~Tab() {}
+
+}  // namespace engine
+}  // namespace blimp