Introduce content::PageState (again).

content/public/common/page_state.cc

+// Copyright (c) 2013 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 "content/public/common/page_state.h"
+
+namespace content {
+
+// static
+PageState PageState::CreateFromEncodedData(const std::string& data) {
+  return PageState(data);
+}
+
+PageState::PageState() {
+}
+
+bool PageState::IsValid() const {
+  return !data_.empty();
+}
+
+bool PageState::Equals(const PageState& other) const {
+  return data_ == other.data_;
+}
+
+const std::string& PageState::ToEncodedData() const {
+  return data_;
+}
+
+PageState::PageState(const std::string& data)
+    : data_(data) {
+  // TODO(darin): Enable this DCHECK once tests have been fixed up to not pass
+  // bogus encoded data to CreateFromEncodedData.
+  //DCHECK(IsValid());
+}
+
+}  // namespace content