Add linux3 support.

'Cause linux2 is so last decade.

TBR=evan@chromium.org
BUG=85845
TEST=

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@88788 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/gclient.py b/gclient.py
index b36f581..4cc4186 100644
--- a/gclient.py
+++ b/gclient.py
@@ -579,6 +579,7 @@
     "unix": "unix",
     "linux": "unix",
     "linux2": "unix",
+    "linux3": "unix",
   }
 
   DEFAULT_CLIENT_FILE_TEXT = ("""\