Make all GPU unittests pass in Python 3

Makes all the unittests under //content/test/gpu pass when run under
Python 3. Additionally, adds vpython3 shebangs to unittest files to
make them run under both Python 2 and 3 during presubmit.

Must be submitted after crrev.com/c/2966012, as otherwise the
unittests can hit a Python 3 incompatibility in Telemetry.

Bug: 1198237
Change-Id: I2252f4c0fe20da6decb9a6e33e6dac32526c5129
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2965142
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#893510}

diff --git a/.vpython3 b/.vpython3
index fbab716..14faab3 100644
--- a/.vpython3
+++ b/.vpython3

@@ -228,3 +228,26 @@
   name: "infra/python/wheels/coverage/${vpython_platform}"
   version: "version:5.5"
 >
+
+# Used by:
+#   //content/test/gpu
+wheel: <
+  name: "infra/python/wheels/pathos/${vpython_platform}"
+  version: "version:0.2.7.chromium.4"
+  not_match_tag <
+    abi: "cp27mu"
+    platform: "manylinux1_i686"
+  >
+  not_match_tag <
+    abi: "cp27mu"
+    platform: "linux_mips64"
+  >
+  not_match_tag <
+    abi: "cp27mu"
+    platform: "linux_armv6l"
+  >
+  not_match_tag <
+    abi: "cp27mu"
+    platform: "linux_armv7l"
+  >
+>