share constructors/enum in both variants of SkReadBuffer

Bug: skia:
Change-Id: I5a90e1f9a9687df2dd2f69d96ac805bf244cec50
Reviewed-on: https://skia-review.googlesource.com/c/179844
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
1 file changed