Reland "Teach Skia to use discardable memory"

Previous change was failing because Android doesn't support
SkDiscardablePixelRef. It is now used only on non-Android platforms.

BUG=318490

Review URL: https://codereview.chromium.org/113753004

git-svn-id: svn://svn.chromium.org/blink/trunk@163917 bbb929c8-8fbe-4397-9dbb-9b2b20218538