This follows the "isolated script test" contract in recipes'
chromium_tests/steps.py. Once the recipes are switched over to run
this new isolate, a lot of GPU-specific code can be deleted from the
tools/build workspace. After the cutover, this new name will be
removed. This is the first step in deleting the GPU recipe entirely.

Tested locally on Linux with GN build, including multiple kinds of
failure injection to ensure that the output JSON is produced as
desired.

BUG=560598
NOTRY=true

Review URL: https://codereview.chromium.org/1472263003

Cr-Commit-Position: refs/heads/master@{#361771}