Profiler based on __gwtStatsEvent (#10960)

* Bootstrap installs a __gwtStatsEvent implementation for tracking GWT's
own bootstrap timing events
* Profiler.enter and Profiler.leave use __gwtStatsEvent
* Profiler data is displayed based on the data tracked by
__gwtStatsEvent

Change-Id: I3db05ca2622aecb745270f01d47cd71648e3bebd