Allow specificying that only target_os should be used in a gclient file

gclient.py

Index: gclient.py
diff --git a/gclient.py b/gclient.py
index a91c61b32bf45253a9bedf9a8f39b909ec317bdb..c3560bdb16e4838cb914b317e4ba8466f76a6675 100644
--- a/gclient.py
+++ b/gclient.py
@@ -55,6 +55,14 @@ Specifying a target OS
 
   Example:
     target_os = [ "android" ]
+
+  If the "target_os_only" key is also present and true, then *only* the
+  operating systems listed in "target_os" will be used. If "target_os" is not
+  present, "target_os_only" has no effect.
+
+  Example:
+    target_os = [ "ios" ]
+    target_os_only = True
 """
 
 __version__ = "0.6.4"
@@ -886,7 +894,10 @@ solutions = [
 
     # Append any target OS that is not already being enforced to the tuple.
     target_os = config_dict.get('target_os', [])
-    self._enforced_os = tuple(set(self._enforced_os).union(target_os))
+    if len(target_os) > 0 and config_dict.get('target_os_only', False):

[M-A Ruel, 2012/11/02 12:07:21]

if target_os and ...
will work just fine.

in fact, I'd prefer it to raise an exception:
if not target_os and config_dict.get('target_os_only', False):
  raise gclient_utils.Error('Can\'t use target_os_only if target_os is not
specified')

Do not assume users won't do stupid things. :)

[stuartmorgan, 2012/11/02 12:36:12]

Good call; done (and test updated to check that it does).

+      self._enforced_os = tuple(set(target_os))
+    else:
+      self._enforced_os = tuple(set(self._enforced_os).union(target_os))
 
     deps_to_add = []
     for s in config_dict.get('solutions', []):