Moved webgl-profiler-api-changes to SLOW category.

Review URL: https://codereview.chromium.org/393223003

BUG=394210

Review URL: https://codereview.chromium.org/393223003

git-svn-id: svn://svn.chromium.org/blink/trunk@179153 bbb929c8-8fbe-4397-9dbb-9b2b20218538