BrowserThemePacks can do IO during shutdown, so delete them on the FILE thread.
BrowserThemePacks can do IO during shutdown, so delete them on the FILE thread.

BUG=61135
TEST=none

Review URL: http://codereview.chromium.org/4213005

git-svn-id: http://src.chromium.org/svn/trunk/src@64478 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 91b58ecd52590d95fc376798a0cda137fe9ddea8
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser