use typedef to name our ID type in SkImageCache

BUG=
R=scroggo@google.com

Review URL: https://codereview.chromium.org/18612003

git-svn-id: http://skia.googlecode.com/svn/trunk@9943 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed