cr: Add support for GN build file generation

tools/cr/cr/commands/prepare.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """A module for the prepare command."""
 
 import cr
 
 
 class PrepareCommand(cr.Command):
@@ skipping 13 matching lines @@
     self.description = ("""
         This does any preparation needed for the output directory, such as
         running gyp.
         """)
 
   def Run(self):
     self.Prepare()
 
   @classmethod
   def UpdateContext(cls):
-    for preparation in PrepareOut.Plugins():
-      preparation.UpdateContext()
+    PrepareOut.GetActivePlugin().UpdateContext()
 
   @classmethod
   def Prepare(cls):
     cls.UpdateContext()
-    for preparation in PrepareOut.Plugins():
-      preparation.Prepare()
+    PrepareOut.GetActivePlugin().Prepare()
 
 
 class PrepareOut(cr.Plugin, cr.Plugin.Type):
   """Base class for output directory preparation plugins.
 
   See PrepareCommand for details.
   """
 
+  SELECTOR = 'CR_GENERATOR'
+
+  @classmethod
+  def AddArguments(cls, parser):
+    parser.add_argument(
+        '--generator', dest=cls.SELECTOR,
+        choices=cls.Choices(),
+        default=None,
+        help='Sets the build file generator to use. ' + \

[petrcermak, 2015/04/17 16:53:14]

Again, I think it would be better to use brackets rather than backslash here.

[Sami, 2015/04/17 16:57:35]

Done.

+             'Overrides %s.' % cls.SELECTOR
+    )
+
   def UpdateContext(self):
     """Update the context if needed.
 
     This is also used by commands that want the environment setup correctly, but
     are not going to call Prepare directly (such as sync)."""
 
   def Prepare(self):
     """All PrepareOut plugins must override this method to do their work."""
     raise NotImplementedError('Must be overridden.')