Revert https://chromiumcodereview.appspot.com/12583004/ - Switch the ASan OSX bots to 10.8
Revert https://chromiumcodereview.appspot.com/12583004/ - Switch the ASan OSX bots to 10.8

The CL has caused strange flakiness of the browser_tests and content_browsertests on the Mac ASan bots.
Since other ASan bots also experience flakiness caused by problems with the swarm client, we switch the OS X
bots back to 10.6 until that's resolved.

BUG=154281
TBR=maruel

Review URL: https://chromiumcodereview.appspot.com/12434012

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@188170 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build