Fix HostResolver crash when MessageLoop is destroyed during asynchronous resolution.
Fix HostResolver crash when MessageLoop is destroyed during
asynchronous resolution.

This should hopefully fix reliability failures introduced in r9312.

TBR=darin
Review URL: http://codereview.chromium.org/19534

git-svn-id: http://src.chromium.org/svn/trunk/src@9317 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 356f6c2700e22d465bf73fbe85aa0f95d8281bcb
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