This depends on a number of factors, including the precise monitor
configuration, so make it easy to get from users.

BUG=678800
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

Review-Url: https://codereview.chromium.org/2786303002
Cr-Commit-Position: refs/heads/master@{#461291}