Create jsondiff tool to capture diffs between two JSON expectations files.

Create jsondiff tool to capture diffs between two JSON expectations files.
This will be used to view diffs before committing rebaselines, or to review
exactly what changed between two baseline revisions in the past.

Part of Step 3 in https://goto.google.com/ChecksumTransitionDetail

R=scroggo@google.com

Committed: https://code.google.com/p/skia/source/detail?r=9878

[epoger, 2013-07-02 18:12:04]

Ready for review at patchset 2.

[epoger, 2013-07-02 18:13:15]

[CCing others who might be interested in knowing what's cooking in
rebaseline-land]

[epoger, 2013-07-02 18:24:32]

https://codereview.chromium.org/18103005/diff/2001/tools/jsondiff.py
File tools/jsondiff.py (right):

https://codereview.chromium.org/18103005/diff/2001/tools/jsondiff.py#newcode13
tools/jsondiff.py:13: and generates an old-vs-new diff dictionary.
For now, this tool just generates a dictionary showing the old-vs-new checksums.
 Once I get this committed, the next steps will be:

1. rebaseline.py: if the existing expectations are stored as a JSON file rather
than image files, generate the new expectations as a JSON file also (just modify
the JSON file in the local checkout)

2. Use ExportBaseVersionOfFile() in
https://code.google.com/p/skia/source/browse/trunk/tools/svn.py , so jsondiff.py
can compare the base version of the JSON expectations file (what was checked out
from the repo) to the locally modified version.  We'll need something similar
for git.

3. Download images and call skdiff to display pixel diffs based on jsondiff's
output

[scroggo, 2013-07-02 18:45:38]

lgtm

[epoger, 2013-07-02 20:22:30]

Committed patchset #2 manually as r9878 (presubmit successful).