Snap time dividers to nearest numbers instead of just dividing the time span...

Snap time dividers to nearest numbers instead of just dividing the time span into max density slices.

https://bugs.webkit.org/show_bug.cgi?id=125081

Reviewed by Joseph Pecoraro.

* UserInterface/TimelineDecorations.js:
(WebInspector.TimelineDecorations.prototype.updateHeaderTimes): Snap the time slice to a nearest number.
(WebInspector.TimelineDecorations.prototype.updateEventMarkers): Rename boundarySpan to timeSpan.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@161893 268f45cc-cd09-0410-ab3c-d52691b4dbfc