LTR/RTL iterateThroughClustersInGlyphsOrder simplified

Change-Id: I778039babc51b60f745a0acff91008d285d94b93
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/284926
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Julia Lavrova <jlavrova@google.com>
4 files changed