Fix return documentation for SkFont::measureText.
Fix return documentation for SkFont::measureText.

There seems to be a copy-paste issue with the documentation for the
return value of SkFont::countText. SkFont::countText returns the number
of glyphs, measureText returns the sum of the default advances.

Change-Id: Id3c9008c20e7b4b8ad4f1d834df19183a9b2bf1e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/530046
Reviewed-by: Derek Sollenberger <d9b232705cad36bae5cfccf14f9650bb292e6c71@google.com>
Commit-Queue: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>

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