Fix a race in proxy_service_unittest.cc that was causing flakiness on purify build-bot.
Fix a race in proxy_service_unittest.cc that was causing flakiness on purify build-bot.

The issue is that the test helper "SyncProxyService" is deleting the underlying ProxyService from the main thread, it should instead be deleted from IO thread.

Deleting from the main thread allows for a small window where deletion of ProxyService can begin while ProxyService::ProcessRequestsQueue() is still executing.

BUG=8738
Review URL: http://codereview.chromium.org/42596

git-svn-id: http://src.chromium.org/svn/trunk/src@12523 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: aa9b5d8b6127e50cb42b0eac638da9fd54c00613
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