Revert changes to startup_metric_utils.cc.

I recently enabled some code on OS X that lets startup_metric_utils.cc set the
thread priority (https://codereview.chromium.org/1620673003/). This is most
likely responsible for a performance regression.

BUG=595043

Review URL: https://codereview.chromium.org/1838173003

Cr-Commit-Position: refs/heads/master@{#384113}