rebaseline_server: delete invalid image files from local disk cache

BUG=skia:2247
NOTREECHECKS=True
NOTRY=True
R=borenet@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/211733004

git-svn-id: http://skia.googlecode.com/svn/trunk@13951 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed