[RPP] Simplify RendererHandler implementation

Working on merging samples with trace events I noted some opportunities
to simplify:

1. Remove code we don't use.
   a. Styles recalc initiator calculation
   b. "Hot" functions calculation

   This is stuff we don't need at the moment. If in the future we need
   to implement it, I think it would be better to do it somewhere else,
   otherwise we risk making the renderer handler a new TimelineModel in
   terms of having all the core timeline code.

2. Store a reference to the event on each RendererEventNode. Instead of
   storing its index in the original trace event array. This way we
   don't have to query through multiple data structures to go from node
   to event.

3. Remove synthetic "totalTime" property in renderer events. It's
   equivalent to the already existing "dur" property.

4. Other small simplifications throughout the code.

Bug: 1449191
Change-Id: Ic8274cd501d22a6a1cab3163ed7c1bb1c38d8e8b
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/4596066
Reviewed-by: Jack Franklin <jacktfranklin@chromium.org>
Commit-Queue: Andres Olivares <andoli@chromium.org>