make render_pictures properly handle empty expectations file

note that this changes the parameters taken by ImageResultsAndExpectations::Parse()

BUG=skia:1942
R=scroggo@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/289063010

git-svn-id: http://skia.googlecode.com/svn/trunk@14779 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed