Simplify subtree (now subsequence) caching

Source/platform/graphics/paint/SubsequenceRecorder.cpp

Index: Source/platform/graphics/paint/SubsequenceRecorder.cpp
diff --git a/Source/platform/graphics/paint/SubsequenceRecorder.cpp b/Source/platform/graphics/paint/SubsequenceRecorder.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..d71c91cd2df8c68ce3613ee1100fc63e409f6472
--- /dev/null
+++ b/Source/platform/graphics/paint/SubsequenceRecorder.cpp
@@ -0,0 +1,57 @@
+// 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.
+
+#include "config.h"
+#include "platform/graphics/paint/SubsequenceRecorder.h"
+
+#include "platform/RuntimeEnabledFeatures.h"
+#include "platform/graphics/GraphicsContext.h"
+#include "platform/graphics/paint/CachedDisplayItem.h"
+#include "platform/graphics/paint/DisplayItemList.h"
+#include "platform/graphics/paint/SubsequenceDisplayItem.h"
+
+namespace blink {
+
+bool SubsequenceRecorder::useCachedSubsequenceIfPossible(GraphicsContext& context, const DisplayItemClientWrapper& client)
+{
+    // The conditions of returning false is not complete for a specific client.

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

Move to .h file, and suggest changing as described in a comment there.

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

Done.

+    // The client must check additional conditions before calling this function.
+
+    if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled())
+        return false;
+
+    ASSERT(context.displayItemList());
+
+    if (context.displayItemList()->displayItemConstructionIsDisabled() || RuntimeEnabledFeatures::slimmingPaintUnderInvalidationCheckingEnabled())
+        return false;
+
+    if (!context.displayItemList()->clientCacheIsValid(client.displayItemClient()))
+        return false;
+
+    context.displayItemList()->createAndAppend<CachedDisplayItem>(client, DisplayItem::CachedSubsequence);
+    return true;
+}
+
+SubsequenceRecorder::SubsequenceRecorder(GraphicsContext& context, const DisplayItemClientWrapper& client)
+    : m_displayItemList(context.displayItemList())
+    , m_client(client)
+{
+    if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled())
+        return;
+
+    ASSERT(m_displayItemList);
+    m_displayItemList->createAndAppend<BeginSubsequenceDisplayItem>(m_client);
+}
+
+SubsequenceRecorder::~SubsequenceRecorder()
+{
+    if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled())
+        return;
+
+    // Don't remove no-op BeginSubsequence/EndSubsequence pairs because we need to
+    // match them later with CachedSubsequences.
+    m_displayItemList->createAndAppend<EndSubsequenceDisplayItem>(m_client);
+}
+
+} // namespace blink