Update and reformat perf.md.

In particular, the following were changed:
 * Reduce wordiness.
 * Run mdformat.
 * Remove information about SCOPED_TRACE(), since that appears to be
   outdated (or maybe just misleading).
 * Replace section on Google-internal script with link to internal
   Chrome A11y page on the topic.

Change-Id: I94e3a47f435f092b225a60ae2e35c06ddba8d2ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1555417
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652853}
1 file changed