Revert of Switch uses of SkChecksum::Compute to Murmur3.

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:

Committed: https://skia.googlesource.com/skia/+/3c2d32b8e27820a6e149d9ded67cbdf2411cc5c9

[mtklein, 2015-11-16 19:01:01]

Created Revert of Switch uses of SkChecksum::Compute to Murmur3.

[commit-bot: I haz the power, 2015-11-16 19:01:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1448023005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1448023005/1

[commit-bot: I haz the power, 2015-11-16 19:01:22]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/3c2d32b8e27820a6e149d9ded67cbdf2411cc5c9