perfetto-ui: Fix slices disappearing bug

The last few slices in a track were sometimes disappearing.
The data arrays were set to the length of totalSlices - which correctly
calculated that the data length should be the count(big slices)
+ count(distinct grouping, depth of slices to be summarized).

What went wrong was that the final data was ordered by ts, percent which
meant that the data was ordered without regard for the depth.
(ordering by ts already accounts for the grouping).

For example data looked like this:
grouping | depth | percent
764 | 1 | 0.6
764 | 2 | 0.2
764 | 1 | 0.1

When creating the final slices - line 1 and 3 would end up being
separate slices, but we want them to be one summarized slice.
This pushed the number of slices to be > totalSlices which was
why the final slices were disappearing.
So the solution is to ensure the data is sorted by depth before
percent.

Bug: 158297138
Change-Id: I6a8f1caee1bf388aba986b8d1c3d904018b7c554