perfetto-ui: Ensure userspace 'busy' slices are accurate
perfetto-ui: Ensure userspace 'busy' slices are accurate

The span join is now partitioned by depth and 'busy' slices now
start at the beginning of the bucket. This ensures busy slices
will never overlap.

Change-Id: Icac6bfdb3649a40d56e5960c189bb667c49e1fc1
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto