move ashmem switching logic to SkDiscardableMemory::Create

R=scroggo@google.com, reed@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/293283002

git-svn-id: http://skia.googlecode.com/svn/trunk@14882 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed