Previously we were using a separate mechanism to capture screen
shots. This would cause very long traces to replay the entire
trace before normal execution. The Maps trace used hundreds of
frames and this was timing out the test step.

Instead we can capture a screenshot after the first time we run
a trace. This will normally not interfere with performance work
because we don't usually capture screenshots and do performance
work at the same time.

Bug: b/172977194
Change-Id: I12cfe2d051d41f1e000a98ac8aef16eb153879c0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2552925Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>