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

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=68429

[nsylvain, 2010-12-07 02:36:33]

I'd have to think a lot more to make sure the math is right, but I assume it is
for now.  A few comments below. LGTM

http://codereview.chromium.org/5607005/diff/1/tools/perf_expectations/make_ex...
File tools/perf_expectations/make_expectations.py (right):

http://codereview.chromium.org/5607005/diff/1/tools/perf_expectations/make_ex...
tools/perf_expectations/make_expectations.py:28: (filename, e.errno,
e.strerror))
i thin it might need more spacing here

http://codereview.chromium.org/5607005/diff/1/tools/perf_expectations/make_ex...
tools/perf_expectations/make_expectations.py:30: if not file:
can this happen?

http://codereview.chromium.org/5607005/diff/1/tools/perf_expectations/make_ex...
tools/perf_expectations/make_expectations.py:46: print >> sys.stderr, ("Error
parsing string: '%s'" % string)
quotes

[chase, 2010-12-07 21:50:03]

Following up in http://codereview.chromium.org/5654003/.