Move the mac_asan trybots from the pool of 10.6 machines to the pool of 10.7 machines.

Move the mac_asan trybots from the pool of 10.6 machines to the pool of 10.7 machines.
Wipe out the mac_asan_alt trybots.

We need the mac_asan bots use at least Xcode 4.4 (otherwise they're unable to operate on ASan dynamic library,
which contains some load commands unsupported by Xcode 4.2), and the easiest way to do so is to switch them to 10.7

BUG=154281, 170629

[Nico, 2013-04-12 19:16:21]

The change itself looks fine. I started a thread with the infra guys about
capacity. Likely ok, but it's better to ask :-) (I cc'd you)

[Nico, 2013-04-21 01:03:46]

lgtm, let's land this. If there are capacity issues, we can always revert.

The try master needs to be restarted to let this have an effect. Saturday or
sunday night should be less busy, so maybe you can ask chrome-troopers for a
restart after landing the CL.

[commit-bot: I haz the power, 2013-04-21 08:37:24]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/glider@chromium.org/14234015/1

[commit-bot: I haz the power, 2013-04-21 08:37:51]

Change committed as 195411