flamegraph: Reduce font size and improve rollover rendering
flamegraph: Reduce font size and improve rollover rendering

- Reduce the font size of the row label font as the monospace
  reads over large. Also reduce the bar height to match and render
  the label in the center of the bar.
- Fix rollover rendering, there where several minor bugs with out
  we determined the width/height of the rollover box, where to
  render the lines in the box etc.

Change-Id: I4eb706bdf4fbd6f6af0ff70b5cc290cd55ae6c59
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto