Use llvm-ar when building with Clang.

build/toolchain/gcc_toolchain.gni

 # Copyright (c) 2013 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.
 
 import("//build/config/android/config.gni")
 import("//build/config/clang/clang.gni")
 import("//build/config/nacl/config.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/v8_target_cpu.gni")
 import("//build/toolchain/cc_wrapper.gni")
@@ skipping 535 matching lines @@
   } else {
     toolprefix = ""
   }
 
   gcc_toolchain(target_name) {
     prefix = rebase_path("$clang_base_path/bin", root_build_dir)
     cc = "$prefix/clang"
     cxx = "$prefix/clang++"
     ld = cxx
     readelf = "${toolprefix}readelf"
-    ar = "${toolprefix}ar"
+    ar = "${prefix}/llvm-ar"
     nm = "${toolprefix}nm"
 
     forward_variables_from(invoker,
                            [
                              "strip",
                              "is_clang_analysis_supported",
                              "enable_linker_map",
                            ])
 
     toolchain_args = {
       if (defined(invoker.toolchain_args)) {
         forward_variables_from(invoker.toolchain_args, "*")
       }
       is_clang = true
     }
   }
 }