Blink GC plugin: Ensure that classes with virtual trace methods always have vtables for their left-…

tools/clang/blink_gc_plugin/tests/non_virtual_trace.txt

Index: tools/clang/blink_gc_plugin/tests/non_virtual_trace.txt
diff --git a/tools/clang/blink_gc_plugin/tests/non_virtual_trace.txt b/tools/clang/blink_gc_plugin/tests/non_virtual_trace.txt
index 8081e8880977ebbbfb7b72d0ad7d78587bafa7bc..a05a94de8975730dab781fee5f94d98a6bad1cb4 100644
--- a/tools/clang/blink_gc_plugin/tests/non_virtual_trace.txt
+++ b/tools/clang/blink_gc_plugin/tests/non_virtual_trace.txt
@@ -1,7 +1,17 @@
+In file included from non_virtual_trace.cpp:5:
+./non_virtual_trace.h:12:1: warning: [blink-gc] Left-most base class 'A' of derived class 'D' must define a virtual trace method.
+class A : public GarbageCollected<A> {
+^
 non_virtual_trace.cpp:13:1: warning: [blink-gc] Class 'C' overrides non-virtual trace of base class 'A'.
 void C::trace(Visitor* visitor)
 ^
 ./non_virtual_trace.h:14:5: note: [blink-gc] Non-virtual trace method declared here:
     void trace(Visitor*);
     ^
-1 warning generated.
+non_virtual_trace.cpp:18:1: warning: [blink-gc] Class 'D' overrides non-virtual trace of base class 'A'.
+void D::trace(Visitor* visitor)
+^
+./non_virtual_trace.h:14:5: note: [blink-gc] Non-virtual trace method declared here:
+    void trace(Visitor*);
+    ^
+3 warnings generated.