At startup, we make a list of all the temp directories that already exist and
then nuke them on a background thread after a five second delay. The five
second delay isn't particularly satisfying, but I think it's wortwhile to get
this work off the critical path of startup.
R=eseidel@chromium.org, eseidel@google.com
Review URL: https://codereview.chromium.org/1228083003 .