Fixed mouse hover over slice
Fixed mouse hover over slice

Previous formula for depth in getSliceIndex was wrong, it
didn't take into account track's padding.

Change-Id: I8990169f603e5845a66afab931589bdfa8fbf78e

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto