blink_gc_plugin: warn of unused trace methods to stack allocated classes.

tools/clang/blink_gc_plugin/tests/stack_allocated.txt

 In file included from stack_allocated.cpp:5:
 ./stack_allocated.h:14:1: warning: [blink-gc] Class 'PartObject' requires a trace method.
 class PartObject {
 ^
 ./stack_allocated.h:17:5: note: [blink-gc] Untraced field 'm_obj' declared here:
     Member<HeapObject> m_obj; // Needs tracing.
     ^
-./stack_allocated.h:26:1: warning: [blink-gc] Class 'HeapObject' contains invalid fields.
+./stack_allocated.h:24:5: warning: [blink-gc] The stack allocated class 'StackObject' provides an unnecessary trace method:
+    void Trace(Visitor* visitor) { visitor->Trace(m_obj); }
+    ^
+./stack_allocated.h:30:1: warning: [blink-gc] Class 'HeapObject' contains invalid fields.
 class HeapObject : public GarbageCollected<HeapObject> {
 ^
-./stack_allocated.h:30:5: note: [blink-gc] Stack-allocated field 'm_part' declared here:
+./stack_allocated.h:34:5: note: [blink-gc] Stack-allocated field 'm_part' declared here:
     StackObject m_part; // Cannot embed a stack allocated object.
     ^
-./stack_allocated.h:34:27: warning: [blink-gc] Stack-allocated class 'DerivedHeapObject' derives class 'HeapObject' which is garbage collected.
+./stack_allocated.h:38:27: warning: [blink-gc] Stack-allocated class 'DerivedHeapObject' derives class 'HeapObject' which is garbage collected.
 class DerivedHeapObject : public HeapObject, public StackObject {
                           ^
-./stack_allocated.h:38:28: warning: [blink-gc] Stack-allocated class 'DerivedHeapObject2' derives class 'HeapObject' which is garbage collected.
+./stack_allocated.h:42:28: warning: [blink-gc] Stack-allocated class 'DerivedHeapObject2' derives class 'HeapObject' which is garbage collected.
 class DerivedHeapObject2 : public HeapObject {
                            ^
-./stack_allocated.h:39:3: warning: [blink-gc] Garbage collected class 'DerivedHeapObject2' is not permitted to override its new operator.
+./stack_allocated.h:43:3: warning: [blink-gc] Garbage collected class 'DerivedHeapObject2' is not permitted to override its new operator.
   STACK_ALLOCATED();
   ^
 ./heap/stubs.h:178:5: note: expanded from macro 'STACK_ALLOCATED'
     __attribute__((annotate("blink_stack_allocated")))      \
     ^
 stack_allocated.cpp:12:1: warning: [blink-gc] Class 'AnonStackObject' contains invalid fields.
 class AnonStackObject : public StackObject {
 ^
 stack_allocated.cpp:14:5: note: [blink-gc] Raw pointer field 'm_obj' to a GC managed class declared here:
     HeapObject* m_obj;
     ^
-6 warnings generated.
+7 warnings generated.