Revert "Revert "SkAdvancedTypefaceMetrics: factor out GlyphToUnicode""

This reverts commit 97c1108607584b6050a6880d6ce22846e4913a92.

Change-Id: Ic3c6addc64ced39766bbee3e10b4d88faf61ba2f
Reviewed-on: https://skia-review.googlesource.com/127021
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
24 files changed