Two malloc+bzero -> calloc.

Two malloc+bzero -> calloc.

I was profiling DM and noticed a couple spots where we malloc then bzero.
These might as well call calloc instead:
   - any time DM itself allocates bitmaps for raster drawing;
   - any time Skia allocates memory for a raster SkSurface.

We could use malloc for opaque surfaces, but it seems simpler to always calloc.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1557713003

Review URL: https://codereview.chromium.org/1557713003