[depot_tools] Add mock lib

Bug:
Change-Id: I1c02188625a42af974744d0c27905f879963b0d2
Reviewed-on: https://chromium-review.googlesource.com/783878Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Mun Yong Jang <myjang@google.com>

third_party/mock/LICENSE.txt

+Copyright (c) 2003-2012, Michael Foord
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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third_party/mock/README.chromium

+Name: mock
+URL: http://www.voidspace.org.uk/python/mock/
+Version: 1.0.0
+License: BSD
+License File: LICENSE.txt
+
+Description:
+mock is a library for testing in Python. It allows you to replace parts of your
+system under test with mock objects and make assertions about how they have been
+used. mock is now part of the Python standard library, available as
+unittest.mock in Python 3.3 onwards.
+mock provides a core Mock class removing the need to create a host of stubs
+throughout your test suite. After performing an action, you can make assertions
+about which methods / attributes were used and arguments they were called with.
+You can also specify return values and set needed attributes in the normal way.
+Additionally, mock provides a patch() decorator that handles patching module and
+class level attributes within the scope of a test, along with sentinel for
+creating unique objects. Mock is very easy to use and is designed for use with
+unittest. Mock is based on the ‘action -> assertion’ pattern instead of
+‘record -> replay’ used by many mocking frameworks. mock is tested on Python
+versions 2.4-2.7, Python 3 plus the latest versions of Jython and PyPy.
+
+Modifications:
+None.
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