Blink GC plugin: require a trace method if a class derives multiple bases that need tracing.

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 because it contains fields that require tracing.
+./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:26:28: warning: [blink-gc] Stack-allocated class 'AnotherStackObject' derives class 'PartObject' which is not stack allocated.
 class AnotherStackObject : public PartObject { // Invalid base.
                            ^
 ./stack_allocated.h:32:1: warning: [blink-gc] Class 'HeapObject' contains invalid fields.
 class HeapObject : public GarbageCollected<HeapObject> {
 ^
 ./stack_allocated.h:36:5: note: [blink-gc] Stack-allocated field 'm_part' declared here:
     StackObject m_part; // Cannot embed a stack allocated object.
     ^
 3 warnings generated.