Disallow heap objects containing unsafe on-heap iterators.

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.
 class HeapObject : public GarbageCollected<HeapObject> {
 ^
 ./stack_allocated.h:30:5: 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.
 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.
 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();
   ^
-./heap/stubs.h:149:5: note: expanded from macro 'STACK_ALLOCATED'
+./heap/stubs.h:184:5: note: expanded from macro 'STACK_ALLOCATED'

[dcheng, 2016/12/20 21:23:46]

I think the test expectation is wrong. This is on line 178 of stubs.h, not line
184, which is causing me issues for rebasing my CL. I'm just going to update
this expectation.

[sof, 2016/12/20 21:30:28]

Thanks, please do. "git cl format" is a mixed blessing.

     __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.