Stop abandoned prerenders from blocking requested prerenders.

When a prerender is abandoned, it's kept around for thirty seconds in
case subsequent navigations (e.g. through redirect chains) end up
using it.

But prerenders in this abandoned state shouldn't block requested
prerenders from launching; instead, if we are at our prerender limit,
we can cancel an abandoned prerender early in order to free up a slot.

As well, do not count abandoned prerenders towards the per-channel
limit, since they didn't originate with the page currently displayed
by that channel.

R=tburkard,davidben
BUG=None

Review URL: https://codereview.chromium.org/26501010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230565 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230565