Return the width of a placeholder in iterateThroughSingleRunByStyles
Return the width of a placeholder in iterateThroughSingleRunByStyles

Also avoid visiting the text run that contains the placeholder
character.

See https://github.com/flutter/flutter/issues/104077

Change-Id: I1c9792f165c7fa971dbf13ba3130f1a787065d0d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/541790
Reviewed-by: Julia Lavrova <66a91ddff0a66e6df510780220f7ed6687af5ca5@google.com>
Commit-Queue: Jason Simmons <345008440c885d4763d50f58e545b9839f60f79f@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia