GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/tests/persistent_no_trace.cpp

Index: tools/clang/blink_gc_plugin/tests/persistent_no_trace.cpp
diff --git a/third_party/WebKit/Source/platform/text/linux/HyphenationLinux.cpp b/tools/clang/blink_gc_plugin/tests/persistent_no_trace.cpp
similarity index 52%
copy from third_party/WebKit/Source/platform/text/linux/HyphenationLinux.cpp
copy to tools/clang/blink_gc_plugin/tests/persistent_no_trace.cpp
index ca2b39fe6cce5b64bfbea81e4f691d92e2a85acc..637b46f0e05d40f681bb587ff9e82c6bf9aba946 100644
--- a/third_party/WebKit/Source/platform/text/linux/HyphenationLinux.cpp
+++ b/tools/clang/blink_gc_plugin/tests/persistent_no_trace.cpp
@@ -2,13 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "platform/text/Hyphenation.h"
+#include "persistent_no_trace.h"
 
 namespace blink {
 
-PassRefPtr<Hyphenation> Hyphenation::platformGetHyphenation(const AtomicString&)
-{
-    return nullptr;
+void HeapObject::trace(Visitor* visitor) {
+    visitor->trace(m_crossThreadPersistent);
+    visitor->trace(m_crossThreadWeakPersistent);
 }
 
-} // namespace blink
+}