Move SkTextBlobTrace from src/core to tools/text

This appears to be only used by tests/benches, so I think it belongs
out of src

Change-Id: I4a4ebb7a8221befcba24a9c7697310e86cb99910
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/816004
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
8 files changed