[wptrunner] Generate image diff for failing reftests

Change-Id: I2afb2ef2fd0c208efd787b51bd80d22cccdee1d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2760937
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Luke Z <lpz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#863787}
diff --git a/.vpython3 b/.vpython3
index de296bc57..e914cf5 100644
--- a/.vpython3
+++ b/.vpython3
@@ -124,4 +124,7 @@
   name: "infra/python/wheels/distro-py2_py3"
   version: "version:1.4.0"
 >
-
+wheel: <
+  name: "infra/python/wheels/pillow/linux-amd64_cp38_cp38"
+  version: "version:8.1.2"
+>