Control the order of gclient spec keys in fetch

Follow the explicit iteration order of the spec keys array instead of relying
on the arbitrary iteration order of the hashtable.

BUG=

Review URL: https://chromiumcodereview.appspot.com/13878009

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@194973 0039d316-1c4b-4281-b951-d872f2087c98

fetch.py

@@ -102,7 +102,7 @@ class GclientGitCheckout(GclientCheckout, GitCheckout):
     assert 'solutions' in self.spec
     keys = ['solutions', 'target_os', 'target_os_only']
     gclient_spec = '\n'.join('%s = %s' % (key, self.spec[key])
-                             for key in self.spec if key in keys)
+                             for key in keys if key in self.spec)
     self.spec['gclient_spec'] = gclient_spec
 
   def exists(self):