GM: include filename extension (.png) of each output file in JSON summary

Doing this so that, once we *do* start writing PDF checksums into the JSON
summary, we'll be able to distinguish those from the PNG checksums.
Otherwise, we could have naming collisions.

R=scroggo@google.com

Review URL: https://codereview.chromium.org/14890017

git-svn-id: http://skia.googlecode.com/svn/trunk@9119 2bbb7eff-a529-9590-31e7-b0007b416f81
17 files changed