Support disabling unit test batching. Use it for angle_end2end_tests

This is a reland of https://codereview.chromium.org/1209243004

Depends on https://codereview.chromium.org/1430633002

TBR=piman, kbr

BUG=501604, 509883

Review URL: https://codereview.chromium.org/1418113010

Cr-Commit-Position: refs/heads/master@{#357083}
3 files changed