For non-opaque SkBitmapDevices, replace malloc-then-zero with calloc.

There seem about a zillion possible ways to slice this.
This adds and uses SkMallocPixelRef::ZeroedPRFactory.
I'm not married to it.

This appears to clear up a hot spot in the benchmark referenced in this bug:
BUG=516426
I'm not confident enough on Windows to declare that definitively yet.

It probably helps all other non-opaque layers too.  Possibly significantly.

So I don't forget, I profiled this:
out/Release/performance_browser_tests.exe  --gtest_filter=TabCapturePerformanceTest.Performance/2 --single-process

No diffs:
https://gold.skia.org/search2?issue=1430593007&unt=true&query=source_type%3Dgm&master=false

TBR=reed@google.com

Review URL: https://codereview.chromium.org/1430593007
3 files changed