mac: Let mac_sdk default to the oldest available SDK that's at least 10.6

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 5b38b2334b2eb5cc2d197d41a0cdb815da72ad6c..ff8bd0b4e77390419875e5e159ccf76c8e3e9c5a 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -643,7 +643,7 @@
     # Chrome normally builds with the Mac OS X 10.6 SDK and sets the
     # deployment target to 10.5.  Other projects, such as O3D, may override
     # these defaults.
-    'mac_sdk%': '10.6',
+    'mac_sdk%': '<!(python mac/find_sdk.py)',

[Mark Mentovai, 2012/07/27 03:31:30]

I wouldn’t want this to impact official builds. Could you make it conditional?

Also, I don’t remember if this will be executed only once the way this is
written, or if we should do pymod_do_main.

[Nico, 2012/07/27 18:01:05]

Explicitly setting mac_sdk still overrides the output of the script, this just
sets the default value. (Also, bots all have a 10.6 SDK, so it'll select 10.6 on
bots. If we're paranoid about bots and don't set mac_sdk in the bot configs yet,
we should set it in the bot configs.)
It doesn't impact gyp_chromium time in a measurable way at least (29.3s with and
without this)

[Mark Mentovai, 2012/07/27 18:18:35]

Nico wrote:
We can’t set mac_sdk in the bot configs, that’s the wrong place to do it. “Which
SDK do I use?” is a property of the branch and the associated code. Bots (even
official ones) build multiple branches simultaneously, and the SDK value needs
to change from time to time (as it did last week). Hard-coding it somewhere in
the bot config makes it unnecessarily difficult to make these changes in the
tree or to run SDK changes through the try server.

Furthermore, “official build” means some special things that’s not necessarily
true of a developer build, and the SDK used for the build is part of that. Those
bots shouldn’t just pick whatever SDK they happen to have handy, hoping that
it’s 10.6. This should remain an explicit setting on those bots. That’s why I
asked for this to be made conditional on officialness.

     'mac_deployment_target%': '10.5',
 
     # The default value for mac_strip in target_defaults. This cannot be