pull mipmap class into its own (private) header

BUG=
R=scroggo@google.com

Review URL: https://codereview.chromium.org/19462007

git-svn-id: http://skia.googlecode.com/svn/trunk/gyp@10161 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed