Fix calculating clusters from start of input.

Still some code hanging on from optimizing for SkTextBlob.

Bug: skia:8420

Change-Id: I947a9ff691fb1fbcb82934c5597e5db11c12f013
Reviewed-on: https://skia-review.googlesource.com/c/192683
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
1 file changed