[cr tool] Adding the prepare command

tools/cr/cr/commands/prepare.py

Index: tools/cr/cr/commands/prepare.py
diff --git a/tools/cr/cr/commands/prepare.py b/tools/cr/cr/commands/prepare.py
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index 0000000000000000000000000000000000000000..57abccef6bbf64be7cbe181710de8bde6f1a1035
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+++ b/tools/cr/cr/commands/prepare.py
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+# 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):
+  """The implementation of the prepare command.
+
+  The prepare command is used to perform  the steps needed to get an output
+  directory ready to use. These should not be the kind of things that need to
+  happen every time you build something, but the rarer things that you re-do
+  only when you get or add new source files, or change your build options.
+  This delegates all it's behavior to implementations of PrepareOut. These will
+  (mostly) be in the cr.actions package.
+  """
+
+  def __init__(self):
+    super(PrepareCommand, self).__init__()
+    self.help = 'Prepares an output directory'
+    self.description = ("""
+        This does any preparation needed for the output directory, such as
+        running gyp.
+        """)
+
+  def Run(self, context):
+    self.Prepare(context)
+
+  @classmethod
+  def Prepare(cls, context):
+    for preparation in PrepareOut.Plugins():
+      preparation.Prepare(context)
+
+
+class PrepareOut(cr.Plugin, cr.Plugin.Type):
+  """Base class for output directory preparation plugins.
+
+  See PrepareCommand for details.
+  """
+
+  def Prepare(self, context):
+    """All PrepareOut plugins must override this method to do their work."""
+    _ = context
+    raise NotImplementedError('Must be overridden.')
+