Set a batch limit of 1 in angle_perftests.

This should hopefully reduce flakiness and unexpected timeouts.

Bug: chromium:1136900
Change-Id: I279ee5f823f32d3ab91021f6fb50203f99982aa7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2479565Reviewed-by: Wenbin Zhang <wenbinzhang@google.com>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818066}

tools/perf/core/bot_platforms.py

@@ -233,6 +233,7 @@ def _angle_perftests(estimated_runtime=1988):
                           flags=[
                               '--test-launcher-retry-limit=0',
                               '--test-launcher-jobs=1',
+                              '--test-launcher-batch-limit=1',
                           ],
                           estimated_runtime=estimated_runtime)