Add support for specifying a branch to the cobalt recipe.

After this change, custom cobalt branches (for example, 'COBALT_1')
can be checked out via:

fetch cobalt --branch=COBALT_1

Change-Id: I8cdff123c070e079e0223c1ed3df21de6263cd19
diff --git a/recipes/cobalt.py b/recipes/cobalt.py
index 5f77e7a..dc9f77f 100644
--- a/recipes/cobalt.py
+++ b/recipes/cobalt.py
@@ -14,7 +14,11 @@
 
   @staticmethod
   def fetch_spec(props):
-    ref = 'remotes/origin/COBALT'
+    branch = 'COBALT'
+    if props.get('branch'):
+      branch = props['branch']
+    ref = 'remotes/origin/%s' % branch
+
     url = 'https://lbshell-internal.googlesource.com/chromium.git@%s' % ref
     solution = { 'name'   :'src',
                  'url'    : url,