gm_json.py: add ability to load JSON file from a string, not just a local file

This will make it easier to read JSON files from new sources (HTTP, etc.)

R=scroggo@google.com

Review URL: https://codereview.chromium.org/15934018

git-svn-id: http://skia.googlecode.com/svn/trunk@9431 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed