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

tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp

 // Copyright 2014 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.
 
 // This clang plugin checks various invariants of the Blink garbage
 // collection infrastructure.
 //
 // Errors are described at:
 // http://www.chromium.org/developers/blink-gc-plugin-errors
 
@@ skipping 17 matching lines @@
     return llvm::make_unique<BlinkGCPluginConsumer>(instance, options_);
   }
 
   bool ParseArgs(const CompilerInstance&,
                  const std::vector<std::string>& args) override {
     for (const auto& arg : args) {
       if (arg == "dump-graph") {
         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 {
         llvm::errs() << "Unknown blink-gc-plugin argument: " << arg << "\n";
         return false;
       }
     }
     return true;
   }
 
  private:
   BlinkGCPluginOptions options_;
 };
 
 static FrontendPluginRegistry::Add<BlinkGCPluginAction> X(
     "blink-gc-plugin",
     "Check Blink GC invariants");