add gm to show miplevels (patchset #2 id:20001 of https://codereview.chromium.org/849173004/)"

fixed memory leak

This reverts commit 04f07fcbfddbe8ca97d29053554a01c66d44ece2.

BUG=skia:
TBR=

Review URL: https://codereview.chromium.org/854163002
1 file changed