Simplify subtree (now subsequence) caching

Source/platform/graphics/paint/SubsequenceRecorder.h

+// Copyright 2015 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 SubsequenceRecorder_h
+#define SubsequenceRecorder_h
+
+#include "platform/graphics/paint/DisplayItemClient.h"
+
+namespace blink {
+
+class DisplayItemList;
+class GraphicsContext;
+
+// Records subsequence information during painting. The recorder's life span covers
+// all painting operations executed during the root of the subsequence's paint method
+// for the paint phase.

[chrishtr, 2015/08/26 23:34:23]

This comment is not quite accurate, because it depends on the use. Maybe just
say that the SubsequenceRecorder records BeginSubsequenceDisplayItem and
EndSubsequenceDisplayItem sentinels at either end of a continguous sequence
of DisplayItems, and supports caching via a CachedDisplayItem with the
CachedSubsequence DisplayItem type.

Also note that useCachedSubsequenceIfPossible is not sufficient to determine
whether a CachedSubsequence can be used. In particular, the client is
responsible for checking that none of the DisplayItemClients that contribute to
the subsequence have been invalidated.

[Xianzhu, 2015/08/26 23:42:36]

The new comment looks great. Thanks.

Done.

+class PLATFORM_EXPORT SubsequenceRecorder {
+public:
+    static bool useCachedSubsequenceIfPossible(GraphicsContext&, const DisplayItemClientWrapper&);
+
+    SubsequenceRecorder(GraphicsContext&, const DisplayItemClientWrapper&);
+    ~SubsequenceRecorder();
+
+private:
+    DisplayItemList* m_displayItemList;
+    DisplayItemClientWrapper m_client;
+};
+
+} // namespace blink
+
+#endif // SubsequenceRecorder_h