nanobench: option to execute directory of TextBlobTraces as benches

Follow on to https://review.skia.org/250259

Change-Id: I794a59dd8227042423c094bd5f7825cfa27da94c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/250756
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
3 files changed