Add a lifecycle state machine for Page

third_party/WebKit/Source/core/frame/FrameLifecycle.cpp

Index: third_party/WebKit/Source/core/frame/FrameLifecycle.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameLifecycle.cpp b/third_party/WebKit/Source/core/frame/FrameLifecycle.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..77b977e7cab40f41120eac14ff64fc263946e61c
--- /dev/null
+++ b/third_party/WebKit/Source/core/frame/FrameLifecycle.cpp
@@ -0,0 +1,29 @@
+// Copyright 2017 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 "core/frame/FrameLifecycle.h"
+
+#include "wtf/Assertions.h"
+
+namespace blink {
+
+FrameLifecycle::FrameLifecycle() : m_state(Attached) {}
+
+void FrameLifecycle::advanceTo(State state) {
+  switch (state) {
+    case Attached:
+    case Detached:
+      // Normally, only allow state to move forward.
+      DCHECK_GT(state, m_state);
+      break;
+    case Detaching:
+      // We can go from Detaching to Detaching since the detach() method can be
+      // re-entered.
+      DCHECK_GE(state, m_state);
+      break;
+  }
+  m_state = state;
+}
+
+}  // namespace blink