GC plugin: remove mixin trace override restriction.

third_party/WebKit/Source/platform/heap/HeapTest.cpp

Index: third_party/WebKit/Source/platform/heap/HeapTest.cpp
diff --git a/third_party/WebKit/Source/platform/heap/HeapTest.cpp b/third_party/WebKit/Source/platform/heap/HeapTest.cpp
index d75c36deb5669fb103b82a34dd69ae696702640a..02fa6ee69b75c3ccc330171f678ce73645bfc95f 100644
--- a/third_party/WebKit/Source/platform/heap/HeapTest.cpp
+++ b/third_party/WebKit/Source/platform/heap/HeapTest.cpp
@@ -4568,11 +4568,17 @@ public:
     MixinA() : m_obj(IntWrapper::create(100)) { }
     DEFINE_INLINE_VIRTUAL_TRACE()
     {
+        s_traceCount++;
         visitor->trace(m_obj);
     }
+
+    static int s_traceCount;
+
     Member<IntWrapper> m_obj;
 };
 
+int MixinA::s_traceCount = 0;
+
 class MixinB : public GarbageCollectedMixin {
 public:
     MixinB() : m_obj(IntWrapper::create(101)) { }
@@ -4663,6 +4669,35 @@ TEST(HeapTest, DerivedMultipleMixins)
     EXPECT_EQ(4, IntWrapper::s_destructorCalls);
 }
 
+class MixinInstanceWithoutTrace : public GarbageCollected<MixinInstanceWithoutTrace>, public MixinA {
+    USING_GARBAGE_COLLECTED_MIXIN(MixinInstanceWithoutTrace);
+public:
+    MixinInstanceWithoutTrace()
+    {
+    }
+};
+
+TEST(HeapTest, MixinInstanceWithoutTrace)
+{
+    // Verify that a mixin instance without any traceable
+    // references inherits the mixin's trace implementation.
+    clearOutOldGarbage();
+    MixinA::s_traceCount = 0;
+    MixinInstanceWithoutTrace* obj = new MixinInstanceWithoutTrace();
+    {
+        Persistent<MixinA> a = obj;
+        preciselyCollectGarbage();
+        EXPECT_EQ(1, MixinA::s_traceCount);
+    }
+    {
+        Persistent<MixinInstanceWithoutTrace> b = obj;
+        preciselyCollectGarbage();
+        EXPECT_EQ(2, MixinA::s_traceCount);
+    }
+    preciselyCollectGarbage();
+    EXPECT_EQ(2, MixinA::s_traceCount);
+}
+
 class GCParkingThreadTester {
 public:
     static void test()