Unpin the Clang version used by the OS X bots.

tools/clang/scripts/update.sh

 #!/usr/bin/env bash
 # Copyright (c) 2012 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 script will check out llvm and clang into third_party/llvm and build it.
 
 # Do NOT CHANGE this if you don't know what you're doing -- see
 # https://code.google.com/p/chromium/wiki/UpdatingClang
 # Reverting problematic clang rolls is safe, though.
 CLANG_REVISION=179138
 # ASan Mac builders are pinned to this revision, see http://crbug.com/170629.
-CLANG_ASAN_MAC_REVISION=170392
+CLANG_ASAN_MAC_REVISION=${CLANG_REVISION}
 
 THIS_DIR="$(dirname "${0}")"
 LLVM_DIR="${THIS_DIR}/../../../third_party/llvm"
 LLVM_BUILD_DIR="${LLVM_DIR}/../llvm-build"
 LLVM_BOOTSTRAP_DIR="${LLVM_DIR}/../llvm-bootstrap"
 CLANG_DIR="${LLVM_DIR}/tools/clang"
 CLANG_TOOLS_EXTRA_DIR="${CLANG_DIR}/tools/extra"
 COMPILER_RT_DIR="${LLVM_DIR}/projects/compiler-rt"
 ANDROID_NDK_DIR="${LLVM_DIR}/../android_tools/ndk"
 STAMP_FILE="${LLVM_BUILD_DIR}/cr_build_revision"
@@ skipping 387 matching lines @@
   # Run a few tests.
   PLUGIN_SRC_DIR="${THIS_DIR}/../plugins"
   "${PLUGIN_SRC_DIR}/tests/test.sh" "${LLVM_BUILD_DIR}/Release+Asserts"
   cd "${LLVM_BUILD_DIR}"
   make check-all
   cd -
 fi
 
 # After everything is done, log success for this revision.
 echo "${CLANG_REVISION}" > "${STAMP_FILE}"