Fix quoting in update_depot_tools.

The update_depot_tools script had a line where a call out
to the test operator ([]) wasn't quoting all of the arguments,
leading to errors if a directory had spaces in the name.

R=dnj@chromium.org, phajdan.jr@chromium.org

Bug:
Change-Id: Ie1f5f51891a2582614ed3d9b16375d2ad522360b
Reviewed-on: https://chromium-review.googlesource.com/598427
Reviewed-by: Daniel Jacques <dnj@chromium.org>
Commit-Queue: Daniel Jacques <dnj@chromium.org>

update_depot_tools

@@ -39,7 +39,7 @@ else
   fi
 fi
 
-if [ -e $base_dir/.disable_auto_update ]; then
+if [ -e "$base_dir/.disable_auto_update" ]; then
   exit
 fi