UI: make counters deal with negative numbers
UI: make counters deal with negative numbers

Fixes the counters frontend logic to deal with negative
values. Also adds a dashed line as a 0 reference, so
negative counters fully zoomed can be distinguished from positive
ones.

Example: https://screenshot.googleplex.com/ubtj7jMeqvR
Trace: fd77261c7ecef2ef37c1b7be5b8ef237c35ddcc7581ef15dc1dadf8da1cb6fb
Change-Id: I3eb632cff9b8d5ca42703f0cbfbaee3aceca694e

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