Put SubRuns in an alloc on GrTextBlob

This improves performance when there are multiple runs.

Bug: chromium:1029972

Change-Id: If32ddb2baf974ee1af7833710bd10a60e39c0169
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/258736
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed