ANGLE would only read one chunk of stdout/stderr data at a
time. We would end up slowing down reading one chunk every
few hundred MS for tests with a very large output stream.

Test: *debug_negative_coverage*
Bug: angleproject:3162
Change-Id: I072cba147a6d86c02a2eda051ff61ed981990798
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2447040Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>