Make sure the asan clang pinning does not pin the layout bots too

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.
@@ skipping 99 matching lines @@
     return 0
   fi
   # 10.8 Chrome Mac ASan Builder.
   if [[ "${HOST}" == "vm672-m1" ]]; then
     return 0
   fi
   # Chrome Mac ASan LKGR.
   if [[ "${HOST}" == "mini11-a1" ]]; then
     return 0
   fi
-  # mac_asan trybots.
-  for num in $(jot - 600 655)
+  # mac_asan trybots. These share machines with the mac_layout bots, so only
+  # pin clang if the slave is used for an _asan build, not for a layout build.
+  if [[ "${PWD}" != *asan* ]]; then
+    return 1
+  fi
+  for num in {600..655}
   do
     if [[ "${HOST}" == "vm${num}-m4" ]]; then
       return 0
     fi
   done
   return 1
 }
 
 # Use older Clang for ASan Mac builds. See http://crbug.com/170629.
 if [[ -n "${is_asan_mac_builder_hackfix}" ]] || on_asan_mac_host; then
@@ skipping 275 matching lines @@
   # Run a few tests.
   PLUGIN_SRC_DIR="${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}"