Visualize profile output in debug toolbar

This adds a new pane for the debug toolbar labeled Profiler.  It groups
together events sent to wfProfileIn/Out and draws a timeline giving a
visual representation of what the page was doing at a particular point
in time. Hovering over a timeline brings up a flyout about the contained
events.

Depends on I08854d80f57286129f19cb14566f84ed3c7fa21c for proper positioning
of the flyout.

Change-Id: I23a0ce4137abcd74ad87967fb263214c08cb9685