Don't start prerenders for prerendered pages queued for deletion.

This caused prerenders that were cancelled for some reason
to be able to launch new prerenders.

BUG=178755
Review URL: https://codereview.chromium.org/12328149

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@185084 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed