Add --writeJsonSummary option to gm

This writes out a JSON file containing expected and actual checksum values for every test.
Next step: allow the same checksum file to be compared against, instead of PNG files.
Review URL: https://codereview.appspot.com/6940053

git-svn-id: http://skia.googlecode.com/svn/trunk@6843 2bbb7eff-a529-9590-31e7-b0007b416f81
15 files changed