Revert of Switch uses of SkChecksum::Compute to Murmur3. (patchset #2 id:20001 of https://codereview.chromium.org/1436973003/ )

Reason for revert:
gotta put back *compute = 0.

Original issue's description:
> Switch uses of SkChecksum::Compute to Murmur3.
>
> SkChecksum::Compute is a very, very poorly distributed hash function.
> This replaces all remaining uses with Murmur3.
>
> The only interesting stuff is in src/gpu.
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/1d024a3c909ae5cefa5e8b339e2b52dc73ee85ac
>
> Committed: https://skia.googlesource.com/skia/+/540e95483d285b555e9b1a73d18c16e7d7c0deba

TBR=bsalomon@google.com,mtklein@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/1448023005
6 files changed