v3 UI sometimes shows same dates twice on the x-axis of time series charts

v3 UI sometimes shows same dates twice on the x-axis of time series charts
https://bugs.webkit.org/show_bug.cgi?id=154210

Reviewed by Chris Dumez.

The bug was caused by the label generation code in TimeSeriesChart.computeTimeGrid never emitting hours.

Use hours instead of dates as labels when the current time's date is same as the previous label's date.
Always include dates before entering this mode to avoid just having hours as labels on the entire x-axis.

* public/v3/components/time-series-chart.js:
(TimeSeriesChart.prototype._renderXAxis): Slightly increase the "average" width of x-axis label.
(TimeSeriesChart.computeTimeGrid): See above. Also assert that the number of labels we generate never
exceeds maxLabels as a sanity check.
(TimeSeriesChart._timeIterators): Added an iterator that increments by two hours for zoomed graphs.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@196556 268f45cc-cd09-0410-ab3c-d52691b4dbfc