Blink GC plugin: Check polymorphic requirements on all GC-derived classes.

tools/clang/blink_gc_plugin/tests/virtual_and_trace_after_dispatch.txt

Index: tools/clang/blink_gc_plugin/tests/virtual_and_trace_after_dispatch.txt
diff --git a/tools/clang/blink_gc_plugin/tests/virtual_and_trace_after_dispatch.txt b/tools/clang/blink_gc_plugin/tests/virtual_and_trace_after_dispatch.txt
index 0c1967c015ef2f06889d38ad3ce165c0e9c3f8e9..fb466967a3350a8292e6e576f31dfdeda53e330e 100644
--- a/tools/clang/blink_gc_plugin/tests/virtual_and_trace_after_dispatch.txt
+++ b/tools/clang/blink_gc_plugin/tests/virtual_and_trace_after_dispatch.txt
@@ -1,8 +1,11 @@
 In file included from virtual_and_trace_after_dispatch.cpp:5:
+./virtual_and_trace_after_dispatch.h:12:1: warning: [blink-gc] Left-most base class 'A' of derived class 'B' must be polymorphic.
+class A : public GarbageCollected<A> {
+^
 ./virtual_and_trace_after_dispatch.h:23:1: warning: [blink-gc] Class 'B' contains or inherits virtual methods but implements manual dispatching.
 class B : public A {
 ^
 ./virtual_and_trace_after_dispatch.h:14:5: note: [blink-gc] Manual dispatch 'trace' declared here:
     void trace(Visitor*);
     ^
-1 warning generated.
+2 warnings generated.