skpdiff: remove default value for output. Diifs can be very big, and fill a hard drive, no sense to generate them if not needed.

R=zachr@google.com

Review URL: https://codereview.chromium.org/20054003

git-svn-id: http://skia.googlecode.com/svn/trunk@10291 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed