Update Blink GC plugin to reflect that Oilpan is now always enabled.

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 16 matching lines @@
       CompilerInstance& instance,
       llvm::StringRef ref) {
     return llvm::make_unique<BlinkGCPluginConsumer>(instance, options_);
   }
 
   virtual bool ParseArgs(const CompilerInstance& instance,
                          const std::vector<std::string>& args) {
     bool parsed = true;
 
     for (size_t i = 0; i < args.size() && parsed; ++i) {
-      if (args[i] == "enable-oilpan") {
-        options_.enable_oilpan = true;
-      } else if (args[i] == "dump-graph") {
+      // TODO(sof): remove this case once a version of the GC plugin
+      // has rolled which has enable-oilpan-always baked in _and_
+      // Blink no longer passes in the option (cf. Source/config.gyp)
+      if (args[i] == "enable-oilpan")
+        continue;
+      if (args[i] == "dump-graph") {
         options_.dump_graph = true;
       } else if (args[i] == "warn-raw-ptr") {
         options_.warn_raw_ptr = true;
       } else if (args[i] == "warn-unneeded-finalizer") {
         options_.warn_unneeded_finalizer = true;
       } else {
         parsed = false;
         llvm::errs() << "Unknown blink-gc-plugin argument: " << args[i] << "\n";
       }
     }
 
     return parsed;
   }
 
  private:
   BlinkGCPluginOptions options_;
 };
 
 static FrontendPluginRegistry::Add<BlinkGCPluginAction> X(
     "blink-gc-plugin",
     "Check Blink GC invariants");