[devil] Add system app replacement wrapper script.

devil/devil/utils/parallelizer.py

Index: devil/devil/utils/parallelizer.py
diff --git a/devil/devil/utils/parallelizer.py b/devil/devil/utils/parallelizer.py
index 35995251c879b130e32f034f0238d4335315c3c6..678066c75077b982ca5ef854c8583188faae2b51 100644
--- a/devil/devil/utils/parallelizer.py
+++ b/devil/devil/utils/parallelizer.py
@@ -118,7 +118,7 @@ class Parallelizer(object):
             o, args=args, kwargs=kwargs,
             name='%s.%s' % (str(d), o.__name__))
          for d, o in zip(self._orig_objs, self._objs)])
-    r._objs.StartAll()  # pylint: disable=W0212
+    r._objs.StartAll()
     return r
 
   def pFinish(self, timeout):
@@ -174,7 +174,7 @@ class Parallelizer(object):
             f, args=tuple([o] + list(args)), kwargs=kwargs,
             name='%s(%s)' % (f.__name__, d))
          for d, o in zip(self._orig_objs, self._objs)])
-    r._objs.StartAll()  # pylint: disable=W0212
+    r._objs.StartAll()
     return r
 
   def _assertNoShadow(self, attr_name):
@@ -200,6 +200,33 @@ class Parallelizer(object):
 class SyncParallelizer(Parallelizer):
   """A Parallelizer that blocks on function calls."""
 
+  def __enter__(self):
+    """Emulate entering the context of |self|.
+
+    Note that this call is synchronous.
+
+    Returns:
+      A Parallelizer emulating the value returned from entering into the
+      context of |self|.
+    """
+    r = type(self)(self._orig_objs)
+    r._objs = [o.__enter__ for o in r._objs]
+    return r.__call__()
+
+  def __exit__(self, exc_type, exc_val, exc_tb):
+    """Emulate exiting the context of |self|.
+
+    Note that this call is synchronous.
+
+    Args:
+      exc_type: the exception type.
+      exc_val: the exception value.
+      exc_tb: the exception traceback.
+    """
+    r = type(self)(self._orig_objs)
+    r._objs = [o.__exit__ for o in r._objs]
+    r.__call__(exc_type, exc_val, exc_tb)
+
   # override
   def __call__(self, *args, **kwargs):
     """Emulate calling |self| with |args| and |kwargs|.