ui: Fix highlighting when selection is far to the left of screen
ui: Fix highlighting when selection is far to the left of screen

If a time region was selected and we zoomed far to the right the greyed
out region eventually ran out. Fix this issue by clamping the lhs of
our fill to 0.

Change-Id: Icf97ecaabb319091699af2e8d471387809bf1685
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto