skia: Added SkDiscardableMemory interface.

Chrome's implementation of SkDiscardableMemory is provided here:
https://codereview.chromium.org/23206002. SkDiscardableMemory is
intended to be used for image caching.

R=reed@google.com, scroggo@google.com, hclam@chromium.org, jamesr@chromium.org
BUG=229120

Author=ernstm@chromium.org

Submitted on behalf of ernstm@chromium.org due to commit queue bug.

Review URL: https://codereview.chromium.org/22950012

git-svn-id: http://skia.googlecode.com/svn/trunk@10797 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed