Mark SkThreadedBMPDevice destructors with override

Very new versions of Clang (5.0 TOT) call out this problem.

Bug: skia:
Change-Id: Ie86d3936c26769c0d3387a81fd2c247ea1272d1c
Reviewed-on: https://skia-review.googlesource.com/18930
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
1 file changed