Ensure that textures exported via SkImage::MakeBackendTextureFromSkImage
have consistent content in their mip map levels.

Bug= chromium:850617

Change-Id: I3ad918aa453bd8e4e625eb145de6ba2a5dab7b0c
Reviewed-on: https://skia-review.googlesource.com/136230
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>