Skip absurdly slow PDF tests on Valgrind bot.

Our Valgrind-with-keepalive CPU bot is still running its first run as I write
this.  It's been going ~48 hours.  'pdf gm fontmgr_iter' finished after ~19
hours.  'pdf image PANO_20121023_214540.jpg' still seems to be running.

After this, the next slowest will be '565 gm fontmgr_iter' at about 37 minutes.

TBR=borenet@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/1003423002