Drop SkRasterPipelineBench N to 15.

This highlights overheads and instruction set switch costs.

At time of writing on my HSW laptop,
   N = 16:  76ns
   N = 15: 291ns

BUG=skia:6289

Change-Id: I01751e8f5ea6cf946e7710822d9bc742712553e9
Reviewed-on: https://skia-review.googlesource.com/8984
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed