Add SkCanvasStack and update the Canvas utilities to use it.

BUG=
R=reed@google.com

Review URL: https://codereview.chromium.org/23865004

git-svn-id: http://skia.googlecode.com/svn/trunk@11081 2bbb7eff-a529-9590-31e7-b0007b416f81
6 files changed