blink_gc_plugin: disallow WeakMember<> fields in off-heap objects.

tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp

Index: tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
diff --git a/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp b/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
index 08572bce6028005ef2aded7036954891ffdf3a06..bd0ec385eef4841c6c47fd535f23f2d37b3f9ac0 100644
--- a/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
+++ b/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
@@ -35,6 +35,8 @@ class BlinkGCPluginAction : public PluginASTAction {
         options_.dump_graph = true;
       } else if (arg == "warn-unneeded-finalizer") {
         options_.warn_unneeded_finalizer = true;
+      } else if (arg == "enable-weak-members-in-unmanaged-classes") {
+        options_.enable_weak_members_in_unmanaged_classes = true;
       } else if (arg == "use-chromium-style-naming") {
         // TODO(dcheng): Remove this once the build no longer passes this flag.
       } else {