Add pyserial to .vpython

Adds pyserial 3.4 to the .vpython file. This is currently only used
by the VR motion-to-photon latency test.

Bug: 812428
Change-Id: I56644689177394dfdba727be41bdf792386e0f2e
Reviewed-on: https://chromium-review.googlesource.com/943923
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540354}

.vpython

@@ -67,6 +67,7 @@ wheel: <
 
 # Used by:
 #   third_party/catapult
+#   chrome/test/vr/perf/latency/run_latency_test.py
 #
 # This version must be compatible with the version range specified by
 # //third_party/catapult/telemetry/telemetry/internal/util/external_modules.py
@@ -146,6 +147,10 @@ wheel: <
     platform: "manylinux1_x86_64"
   >
 >
+wheel: <
+  name: "infra/python/wheels/pyserial-py2_py3"
+  version: "version:3.4"
+>
 wheel: <
   name: "infra/python/wheels/scipy/${vpython_platform}"
   version: "version:0.19.0"