subprocess2: cache the string-escape codec at load time.

This is a speculative fix for subprocess being unable to find
string-escape later in execution; the theory being that something
(unclear what) is interfering w/ codecs' lookup logic and that calling
lookup earlier would allow string-escape to be cached before any such
interference. See crbug.com/912292#c2 for more.

Bug: 912292
Change-Id: I0abdd394253c9a4984db7c6c81d044087edc68fd
Reviewed-on: https://chromium-review.googlesource.com/c/1363753
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>

subprocess2.py

@@ -8,6 +8,7 @@ In theory you shouldn't need anything else in subprocess, or this module failed.
 """
 
 import cStringIO
+import codecs
 import errno
 import logging
 import os
@@ -17,6 +18,9 @@ import sys
 import time
 import threading
 
+# Cache the string-escape codec to ensure subprocess can find it later.
+# See crbug.com/912292#c2 for context.
+codecs.lookup('string-escape')
 
 # Constants forwarded from subprocess.
 PIPE = subprocess.PIPE