Remove references to no-longer-existing perf_av master.

The master got removed in https://codereview.chromium.org/115363004/

BUG=224072
NOTRY=true

Review URL: https://codereview.chromium.org/99433008

git-svn-id: svn://svn.chromium.org/blink/trunk@163985 bbb929c8-8fbe-4397-9dbb-9b2b20218538