service: Fix tracing service unittest build for chromium.
service: Fix tracing service unittest build for chromium.

The usual uint vs int comparison...

Change-Id: I7eec4e31fb1020b64c00dd211e36cd0e84f4b532
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto