Add  SkPurgeGlobalDiscardableMemoryPool, needed to replace SkImageRef

BUG=skia:2389
R=reed@google.com, scroggo@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/298423002

git-svn-id: http://skia.googlecode.com/svn/trunk@14925 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed