Trim the fat off SkPMFloat bench.

Trim the fat off SkPMFloat bench.

This bench was ~75% overhead, ~25% good bench.  It is now just about the
opposite: about 30% of the runtime is loop and random number overhead, and
about 70% of the time is spent doing SkPMColor <-> SkPMFloat work.

BUG=skia:

NOPRESUBMIT=true

Committed: https://skia.googlesource.com/skia/+/60ff4582ae76aa131aeb20a3e16c89fa3a40a41c

[mtklein_C, 2015-03-03 02:06:02]

mtklein@chromium.org changed reviewers:
+ reed@google.com

[reed1, 2015-03-03 15:05:39]

lgtm

[mtklein, 2015-03-03 15:06:46]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-03-03 15:07:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/968133005/1

[commit-bot: I haz the power, 2015-03-03 15:07:05]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-03 15:07:06]

Presubmit check for 968133005-1 failed and returned exit status 1.

Traceback (most recent call last):
  File "/b/infra_internal/commit_queue/verification/presubmit_shim.py", line 34,
in <module>
    sys.exit(presubmit_support.Main(argv))
AttributeError: 'module' object has no attribute 'Main'

[mtklein, 2015-03-03 15:24:08]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-03-03 15:25:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/968133005/1

[commit-bot: I haz the power, 2015-03-03 15:25:04]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-03 15:25:05]

Presubmit check for 968133005-1 failed and returned exit status 1.

Traceback (most recent call last):
  File "/b/infra_internal/commit_queue/verification/presubmit_shim.py", line 34,
in <module>
    sys.exit(presubmit_support.Main(argv))
AttributeError: 'module' object has no attribute 'Main'

[mtklein, 2015-03-03 15:47:37]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-03-03 15:48:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/968133005/1

[commit-bot: I haz the power, 2015-03-03 16:03:30]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/60ff4582ae76aa131aeb20a3e16c89fa3a40a41c