Use SkImage::textureSize instead of GrDirectContext::ComputeImageSize.

This will allow Skia to remove the former, which in turn will assist
in some internal Skia cleanup related internal queries of texture
proxies from SkImages.

Bug: skia:11208
Change-Id: Icdc92c7570f30675c313f4ead696816127bf3965
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2693247


Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#854825}