Add an Allocator interface that indicates if memory is zero init

This is the first step in a three part change:
(1) Skia: Add SkBRDAllocator.
(2) Android: Make JavaPixelAllocator and RecyclingClippingPixelAllocator
             implement SkBRDAllocator.
(3) Skia: Change SkBitmapRegionDecoder to use SkBRDAllocator and take
          advantage of zero allocated memory when possible.

BUG=skia:

Review URL: https://codereview.chromium.org/1423253004
1 file changed