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

tools/clang/blink_gc_plugin/BlinkGCPluginOptions.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TOOLS_BLINK_GC_PLUGIN_BLINK_GC_PLUGIN_OPTIONS_H_
 #define TOOLS_BLINK_GC_PLUGIN_BLINK_GC_PLUGIN_OPTIONS_H_
 
 #include <set>
 #include <string>
 #include <vector>
 
 struct BlinkGCPluginOptions {
   bool dump_graph = false;
 
   // If |true|, emit warning for class types which derive from from
   // GarbageCollectedFinalized<> when just GarbageCollected<> will do.
   bool warn_unneeded_finalizer = false;
 
+  // Member<T> fields are only permitted in managed classes,
+  // something CheckFieldsVisitor verifies, issuing errors if
+  // found in unmanaged classes. WeakMember<T> should be treated
+  // the exact same, but CheckFieldsVisitor was missing the case
+  // for handling the weak member variant until crbug.com/724418.
+  //
+  // We've default-enabled the checking for those also now, but do
+  // offer an opt-out option should enabling the check lead to
+  // unexpected (but wanted, really) compilation errors while
+  // rolling out an updated GC plugin version.
+  //
+  // TODO(sof): remove this option once safely rolled out.
+  bool enable_weak_members_in_unmanaged_classes = false;
+
   std::set<std::string> ignored_classes;
   std::set<std::string> checked_namespaces;
   std::vector<std::string> ignored_directories;
 };
 
 #endif  // TOOLS_BLINK_GC_PLUGIN_BLINK_GC_PLUGIN_OPTIONS_H_