Switch angle_perftests to histograms

Whitelists angle_perftests for automatic conversion from chartjson to
histograms before uploading to the perf dashboard. The tests were
updated to be compatible with histogram conversion in
https://chromium-review.googlesource.com/c/angle/angle/+/1762360.

Bug: 924618, 923564
Change-Id: Idf14f7fee3f8492c2ef9457a858cea1f43a0f5dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1769229Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690441}

tools/perf/process_perf_results.py

@@ -53,6 +53,7 @@ DATA_FORMAT_UNKNOWN = 'unknown'
 # problems. So, only perform the conversion on tests that are whitelisted and
 # are okay with potentially encountering issues.
 GTEST_CONVERSION_WHITELIST = [
+  'angle_perftests',
   'xr.vr.common_perftests',
 ]