PaintMeasureTextTest: only pause/resume not_cached benchmarks

Pausing and resuming the benchmark timer isn't needed unless we plan to
flush the text layout cache.  The cached benchmark runs are so fast on
high-end devices, the overhead of these extra calls can be significant
compared to the actual code being measured.

Test: run PaintMeasureTextTest on sailfish

Bug: 69634871
Change-Id: If8246154e50451d82ac96e026036aa4a42eaed8d
Signed-off-by: Greg Hackmann <ghackmann@google.com>