Added an env variable to hide "Depot tools has been updated..." message

update_depot_tools

 #!/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 try to sync the bootstrap directories and then defer control.
 
 if [ "$USER" == "root" ];
 then
   echo Running depot tools as root is sad.
@@ skipping 128 matching lines @@
 if [ "X$DEPOT_TOOLS_UPDATE" != "X0" -a -e "$base_dir/.svn" ]
 then
   # Update the root directory to stay up-to-date with the latest depot_tools.
   BEFORE_REVISION=$(get_svn_revision)
   if echo $* | grep -e --force > /dev/null; then
     "$SVN" -q revert -R "$base_dir"
   fi
   "$SVN" -q up "$base_dir"
   AFTER_REVISION=$(get_svn_revision)
   if [[ "$BEFORE_REVISION" != "$AFTER_REVISION" ]]; then
-    echo "Depot Tools has been updated to revision $AFTER_REVISION." 1>&2
+    if [ -z "$DEPOT_TOOLS_HIDE_UPDATED_MESSAGE" ]; then
+      echo "Depot Tools has been updated to revision $AFTER_REVISION." 1>&2
+    fi
   fi
 fi
 
 find "$base_dir" -iname "*.pyc" -exec rm {} \;