[chromedriver] Run run_webdriver_tests.py under python3

This CL converts the webdriver_wpt_tests target to be a
script_test, which is py3 by default. Changes are made
as necessary to fix py3 issues:

  * Removed use of hash() in run_webdriver_tests.py to pick
    which tests to run as a shard, because hash() is not
    deterministic between different python runs (so shards would
    overlap).
  * Fixed non-decoded subprocess output in xvfb.py
  * Fixed direct comparison of sys.platform in xvfb.py (one should
    use startswith as the exact string may change).

This also marks one test as failing - previously it seems that
this test was just erroring in a way that caused it to be dropped
entirely (!). With Python 3 it is now running but fails.

Bug: 1189610
Change-Id: I541d22c8ed055386072fda57d0a72f47b8c4d31e
Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2773059
Reviewed-by: Shengfa Lin <shengfa@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Luke Z <lpz@chromium.org>
Commit-Queue: Luke Z <lpz@chromium.org>
Auto-Submit: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864632}
diff --git a/.vpython3 b/.vpython3
index e914cf5..26bd1e5 100644
--- a/.vpython3
+++ b/.vpython3
@@ -128,3 +128,46 @@
   name: "infra/python/wheels/pillow/linux-amd64_cp38_cp38"
   version: "version:8.1.2"
 >
+
+# Used by:
+#   chrome/test/chromedriver/test/run_webdriver_tests.py
+
+wheel <
+  name: "infra/python/wheels/pytest-py2_py3"
+  version: "version:4.1.1"
+>
+
+wheel <
+  name: "infra/python/wheels/attrs-py2_py3"
+  version: "version:18.2.0"
+>
+
+wheel <
+  name: "infra/python/wheels/six-py2_py3"
+  version: "version:1.15.0"
+>
+
+wheel <
+  name: "infra/python/wheels/more-itertools-py2_py3"
+  version: "version:4.1.0"
+>
+
+wheel <
+  name: "infra/python/wheels/pluggy-py2_py3"
+  version: "version:0.8.1"
+>
+
+wheel <
+  name: "infra/python/wheels/py-py2_py3"
+  version: "version:1.5.3"
+>
+
+wheel <
+  name: "infra/python/wheels/funcsigs-py2_py3"
+  version: "version:1.0.2"
+>
+
+wheel: <
+  name: "infra/python/wheels/atomicwrites-py2_py3"
+  version: "version:1.3.0"
+>