Shutdown multiprocess pool after running tests to avoid _finalizer spew on Windows

Can't just let the GC collect the pool on Windows.

R=tandrii@chromium.org
BUG=618726

Review-Url: https://codereview.chromium.org/2296723006

presubmit_support.py

@@ -563,6 +563,11 @@ class InputApi(object):
       msgs.extend(map(CallCommand, tests))
     return [m for m in msgs if m]
 
+  def ShutdownPool(self):
+    self._run_tests_pool.close()
+    self._run_tests_pool.join()
+    self._run_tests_pool = None
+
 
 class _DiffCache(object):
   """Caches diffs retrieved from a particular SCM."""
@@ -1448,6 +1453,8 @@ class PresubmitExecuter(object):
     else:
       result = ()  # no error since the script doesn't care about current event.
 
+    input_api.ShutdownPool()
+
     # Return the process to the original working directory.
     os.chdir(main_path)
     return result

tests/presubmit_unittest.py

@@ -1253,6 +1253,7 @@ class InputApiUnittest(PresubmitTestsBase):
         'ReadFile',
         'RightHandSideLines',
         'ServerPaths',
+        'ShutdownPool',
         'basename',
         'cPickle',
         'cpplint',