Replace Thinkness with Thickness.

Replace Thinkness with Thickness.

The FontMetricsFlags::kUnderlineThinknessIsValid_Flag has a typo in it.
Fortunately no user currently uses this enumeration value by name but
instead uses the getter so the name can be updated.

BUG=skia:6174

Change-Id: I810260d826482de1da8876cd9739d24b3bfb0f95
Reviewed-on: https://skia-review.googlesource.com/10050
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>