Implement PaintWorkletGlobalScope#registerPaint() for the CSS Paint API

third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.h

Index: third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.h
diff --git a/third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.h b/third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.h
new file mode 100644
index 0000000000000000000000000000000000000000..05da0e7a6df843e58327ed9bba59c5c2e4ad95ef
--- /dev/null
+++ b/third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.h
@@ -0,0 +1,35 @@
+// 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.
+
+#ifndef CSSPaintDefinition_h
+#define CSSPaintDefinition_h
+
+#include "bindings/core/v8/ScopedPersistent.h"
+#include "platform/geometry/IntSize.h"
+#include "platform/heap/Handle.h"
+#include <v8.h>
+
+namespace blink {
+
+class ScriptState;
+
+class CSSPaintDefinition final : public RefCountedWillBeGarbageCollectedFinalized<CSSPaintDefinition> {
+public:
+    static PassRefPtrWillBeRawPtr<CSSPaintDefinition> create(ScriptState*, v8::Local<v8::Function> constructor, v8::Local<v8::Function> paint);
+    virtual ~CSSPaintDefinition();
+
+    DEFINE_INLINE_TRACE() { };
+
+private:
+    CSSPaintDefinition(ScriptState*, v8::Local<v8::Function> constructor, v8::Local<v8::Function> paint);
+
+    RefPtr<ScriptState> m_scriptState;
+
+    ScopedPersistent<v8::Function> m_constructor;
+    ScopedPersistent<v8::Function> m_paint;

[haraken, 2016/03/29 04:26:15]

Would you elaborate on why these ScopedPersistents don't cause leaks? For
example, isn't it possible that the following cycle is created?

  CSSPaintDefinition ==(ScopedPersistent)=> v8::Function ==(arbitrary
reference)=> WorkletGlobalScope's wrapper ==(Oilpan's Persistent)=>
WorkletGlobalScope ==(Oilpan's Member)=> CSSPaintDefinition

I guess we need to have a way to explicitly break the Oilpan's Member from the
WorkletGlobalScope to the CSSPaintDefinition when the WorkletGlobalScope stops
(or something). Otherwise, WorkletGlobalScope's wrapper won't be collected
forever.

[ikilpatrick, 2016/03/30 19:12:50]

Yup that cycle was being created. Thanks for noticing.

CustomElements stores the definitions on the V8PerContextData object which gets
disposed of before the window in that case. This clears the (strong)
ScopedPersistent references.

I've changed patch to do the same. Create a new Object "CSSPaintBinding" which
has strong ScopedPersistents, this is stored off the V8PerContextData.

The ScopedPersistents in CSSPaintDefinition now have weak ScopedPersistents.

I wrote a test case (PaintWorkletTest.cpp) which tests for the memory leak.
PTAL.

+};
+
+} // namespace blink
+
+#endif // CSSPaintDefinition_h