MockTestCase - StartPatcher and PatchObject returns mock.

Have StartPatcher() return the PartialMock/mock.Mock instance, for easy
assignment. i.e.,

self.gs_mock = self.StartPatcher(gs_unittest.GSContextMock())

BUG=None
TEST=unittests

Change-Id: I755acb82e5818c9feed32abf1fd628fca41646b5
Reviewed-on: https://gerrit.chromium.org/gerrit/41801
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>
Commit-Queue: Ryan Cui <rcui@chromium.org>
diff --git a/scripts/cros_generate_sysroot_unittest.py b/scripts/cros_generate_sysroot_unittest.py
index 90801c5..8f326ea 100755
--- a/scripts/cros_generate_sysroot_unittest.py
+++ b/scripts/cros_generate_sysroot_unittest.py
@@ -55,8 +55,7 @@
 class OverallTest(cros_test_lib.MockTempDirTestCase):
 
   def setUp(self):
-    self.cg_mock = CrosGenMock()
-    self.StartPatcher(self.cg_mock)
+    self.cg_mock = self.StartPatcher(CrosGenMock())
 
   def testTarballGeneration(self):
     """End-to-end test of tarball generation."""