Don't keep "outResultsFile" open in nanobench

There is a bug on Pixel and Pixel2 devices where the program
eventually terminates with a non-zero exit code. Closing the
outResultsFile between JSON flushes seems to fix it (for whatever
reason).

Bug: b/143074513
Change-Id: I935e982e88758fda19292129c8031f8501cca615
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/249821
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
4 files changed