fix skpdiff viewer bug when using relative paths

BUG=skia:1463
R=djsollen@google.com

Review URL: https://codereview.chromium.org/21601002

git-svn-id: http://skia.googlecode.com/svn/trunk@10515 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed