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

This is needed to prevent auto_deploy.py in CQ send alert each time depot_tools
get's updated as cron job will send alert on any output.

R=stip@chromium.org
CC=akuegel@chromium.org
BUG=475049

Review URL: https://codereview.chromium.org/1073113004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294778 0039d316-1c4b-4281-b951-d872f2087c98

update_depot_tools

@@ -146,7 +146,9 @@ then
   "$SVN" -q up "$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