run_perf_tests: Reduce warmup trials from 3 to 2.

This is an attempt to reduce utilization on the bots, which
are now taking too long and causing errors.

Bug: angleproject:7738
Change-Id: I1c576b2d0989240482d332712f7d93841e933902
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3937051
Reviewed-by: Yuxin Hu <yuxinhu@google.com>
Commit-Queue: Yuxin Hu <yuxinhu@google.com>
Auto-Submit: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

src/tests/run_perf_tests.py

@@ -44,7 +44,7 @@ DEFAULT_TRIALS = 3
 DEFAULT_MAX_ERRORS = 3
 
 # These parameters condition the test warmup to stabilize the scores across runs.
-DEFAULT_WARMUP_TRIALS = 3
+DEFAULT_WARMUP_TRIALS = 2
 DEFAULT_TRIAL_TIME = 5
 
 # Test expectations