Sort traces on the plot so anything with a corresponding _ref trace is plotte...

Sort traces on the plot so anything with a corresponding _ref trace is plotted
first. This will help keep the colors consistent. For example, when a graph
has "t", "t_ref", and "gears" plots, a strictly alphabetical sorting puts
the "gears" plot first, when people are used to seeing t and t_ref in the first
two trace colors.

This is a slightly suboptimal solution. Better would be to incorporate the
"important" marker that process_log.py gets from the test executable. But that
requires a more substantial reworking, and is deferred to another day.

BUG=7704
TEST=See t and t_ref in blue and yellow on a 3-trace startup plot such as
  http://build.chromium.org/buildbot/perf/xp-release-single-core/startup/report.html?history=150

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=10124

[Pam (message me for reviews), 2009-02-20 02:06:41]

Ojan: please review my clumsy JS

Peter: FYI and "UI" review, since you reported the bug

[ojan, 2009-02-20 02:46:41]

LGTM

http://codereview.chromium.org/20527/diff/1/2
File tools/buildbot/perf/dashboard/ui/generic_plotter.html (right):

http://codereview.chromium.org/20527/diff/1/2#newcode197
Line 197: function trace_compare(name1, name2) {
There's a lot of places where you do the same thing twice in a row here. Might
be more readable to make helper functions for those things. That said, it's
probably not worth the effort. :)