rebaseline.py: set text mimetype for all .json files, so text diffs work 
BUG=https://code.google.com/p/skia/issues/detail?id=1442
R=fmalita@google.com

Review URL: https://codereview.chromium.org/20017009

git-svn-id: http://skia.googlecode.com/svn/trunk@10311 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed