SkResourceCache::GetAllocator() index8 and other color types handling

Unit tests added to check all color types in SkOneShotDiscardablePixelref

SkResourceCacheTest seems to have been allways using default Heap Allocator.
 Fixed so that it uses private (not global that is) SkDiscardableMemory.

BUG=skia:4355

Review URL: https://codereview.chromium.org/1426753006
3 files changed