Add GCAPI to the GN build.

build/toolchain/win/midl.gni

 # 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.
 
 assert(is_win)
 
 import("//build/config/win/visual_studio_version.gni")
 
-# This template defines a rule to invoke the MS IDL compiler.
+# This template defines a rule to invoke the MS IDL compiler. The generated
+# source code will be compiled and linked into targets that depend on this.
 #
 # Parameters
 #
 #   sources
 #      List of .idl file to process.
 #
 #   out_dir (optional)
 #       Directory to write the generated files to. Defaults to target_gen_dir.
 #
 #   deps (optional)
@@ skipping 74 matching lines @@
     sources = process_file_template(invoker.sources,
                                     [ "$out_dir/$interface_identifier_file" ])
 
     public_deps = [
       ":$action_name",
     ]
 
     configs += [ "//build/config/win:midl_warnings" ]
   }
 }