Use vpython for GPU integration tests.

Most developers use this harness when running the tests locally, and
they should all have depot_tools' vpython on their PATH. This will
help ensure that they get the hermetic Python that the bots use.

This won't have any effect on the bots since they invoke vpython
earlier, and run more wrapper scripts before getting to this one.

TBR=iannucci@chromium.org, ynovikov@chromium.org

Bug: 716241
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I820ae66b0232387328c75f937c241050f8776c59
Reviewed-on: https://chromium-review.googlesource.com/956553Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541946}

content/test/gpu/run_gpu_integration_test.py

-#!/usr/bin/env python
+#!/usr/bin/env vpython
 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.