Add script to update perf_expectations.json.
Add script to update perf_expectations.json.

This script reads perf_expectations.json for revision
ranges, downloads the appropriate trace data, calculates
the improve/regress ranges, then writes the updated
values back to perf_expectations.json.

BUG=none
TEST=make_expectations.py downloads updated results,
writes correct perf_expectations.json file

Review URL: http://codereview.chromium.org/5607005

git-svn-id: http://src.chromium.org/svn/trunk/src@68429 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 0df47100d06ef7a466e716eb5b9333d8a53690c9
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser