Chart status should always be computed against prior values

Chart status should always be computed against prior values
https://bugs.webkit.org/show_bug.cgi?id=157014

Reviewed by Darin Adler.

Compare the current value against the last baseline or target value that appear before the current value in time
so that the comparison stay the same even when new baseline and target values are reported. Also include the compared
baseline or target value in the label for clarity.

* public/v3/components/chart-status-view.js:
(ChartStatusView.prototype._computeChartStatus):
(ChartStatusView.prototype._computeChartStatus.labelForDiff):
(ChartStatusView.prototype._findLastPointPriorToTime): Extracted from _relativeDifferenceToLaterPointInTimeSeries.
Now finds the last point before the current point's time if there is any, or the last point in baseline / target.
(ChartStatusView.prototype._relativeDifferenceToLaterPointInTimeSeries): Deleted.
* public/v3/models/metric.js:
(Metric.prototype.makeFormatter): Don't use SI units for unit-less metrics.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@200090 268f45cc-cd09-0410-ab3c-d52691b4dbfc