Separate display item clients for negative and normal/positive z-order children

third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h

Index: third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h
diff --git a/third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h b/third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h
index 3ac0892254400cae02cc177e05f224df4ca63c20..b9762fff5a8cc0be5b0958adca9caed03e9de3e5 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h
+++ b/third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h
@@ -12,16 +12,40 @@
 namespace blink {
 
 class DisplayItemClientInternalVoid;
-using DisplayItemClient = const DisplayItemClientInternalVoid*;
+using DisplayItemClientObject = const DisplayItemClientInternalVoid*;
 
-inline DisplayItemClient toDisplayItemClient(const void* object) { return static_cast<DisplayItemClient>(object); }
+const unsigned kMaxSubDisplayItemClientId = (1 << 6) - 1;
+
+struct DisplayItemClient {
+    // subClientId allows multiple display item client per object.
+    // This is useful when we need to cache the display item separately for each sub clients.
+    DisplayItemClient(const void* inObject, unsigned inSubClientId = 0)
+        : object(static_cast<DisplayItemClientObject>(inObject))
+        , subClientId(inSubClientId)
+    {
+        ASSERT(inSubClientId <= kMaxSubDisplayItemClientId);
+    }
+
+    bool operator==(const DisplayItemClient& other) const
+    {
+        return object == other.object && subClientId == other.subClientId;
+    }
+
+    bool operator!=(const DisplayItemClient& other) const
+    {
+        return !(*this == other);
+    }
+
+    DisplayItemClientObject object;
+    size_t subClientId;
+};
 
 // Used to pass DisplayItemClient and debugName() (called only when needed) from
 // core/layout module etc. to platform/paint module.
 // The instance must not out-live the object. Long-time reference to a client must
 // use DisplayItemClient.
 class PLATFORM_EXPORT DisplayItemClientWrapper {
-    DISALLOW_NEW(); // Allow allocated in stack or in another object only.
+    DISALLOW_NEW_EXCEPT_PLACEMENT_NEW(); // Allow allocated in stack or in another object only.
 public:
     template <typename T>
     DisplayItemClientWrapper(const T& object)
@@ -30,6 +54,15 @@ public:
         , m_debugNameInvoker(&invokeDebugName<T>)
     { }
 
+    // Allows one object to create multiple sub display item clients. Use like:
+    // DisplayItemClientWrapper(object, toSubDisplayItemClient<SubClientId>(&object), functionToGetDebugNameOfSubClient)
+    template <typename T>
+    DisplayItemClientWrapper(const T& object, DisplayItemClient displayItemClient, String (*debugNameInvoker)(const T&))
+        : m_displayItemClient(displayItemClient)
+        , m_object(reinterpret_cast<const GenericClass&>(object))
+        , m_debugNameInvoker(reinterpret_cast<DebugNameInvoker>(debugNameInvoker))
+    { }
+
     DisplayItemClientWrapper(const DisplayItemClientWrapper& other)
         : m_displayItemClient(other.m_displayItemClient)
         , m_object(other.m_object)
@@ -46,9 +79,9 @@ private:
     template <typename T>
     static String invokeDebugName(const GenericClass& object) { return reinterpret_cast<const T&>(object).debugName(); }
 
+    using DebugNameInvoker = String (*)(const GenericClass&);
     DisplayItemClient m_displayItemClient;
     const GenericClass& m_object;
-    using DebugNameInvoker = String(*)(const GenericClass&);
     DebugNameInvoker m_debugNameInvoker;
 };