mojo: load ascii codec before importing jinja2.

This is a speculative fix for pickle failing to find the ascii codec
when jinja2 goes to import it. See crbug.com/912292#c2 for prior theory.

Bug: 997598
Change-Id: Ie52b72fbe7cbefeb58ea6dc0aaafa73e1d125bbf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1771554Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690436}

mojo/public/tools/bindings/pylib/mojom/generate/template_expander.py

@@ -4,9 +4,14 @@
 
 # Based on third_party/WebKit/Source/build/scripts/template_expander.py.
 
+import codecs
 import os.path
 import sys
 
+# Cache the ascii codec to ensure pickle can find it at jinja2 import time.
+# See https://crbug.com/997598#c5 for context.
+codecs.lookup('ascii')
+
 _current_dir = os.path.dirname(os.path.realpath(__file__))
 # jinja2 is in chromium's third_party directory
 # Insert at front to override system libraries, and after path[0] == script dir