update size GrTextBlob size calculation

Update calculating GrTextBlob size to reflect different VertexData sizes.

Change-Id: I8127c5ec5b6fdc3c7722cacd316cade5e467b4b9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/307220
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed