Fix Flate Test, also cleanup

Fix Flate Test, also cleanup

-   Remove SkData double-unref to fix assert.
-   Re-enable the test with DEBUG (was disabled due to double-unref assert).
-   SkRandom instead of rand() for thread-safety.
-   General clean up of the test for readability.

Motivation: I am investigating turning on SkFlate::HaveFlate on Windows.

Committed: https://skia.googlesource.com/skia/+/bc677c801523034fd68261e843db597c6625a8bc

[hal.canary, 2014-08-13 13:33:59]

ptal

[hal.canary, 2014-08-13 13:34:02]

The CQ bit was checked by halcanary@google.com

[commit-bot: I haz the power, 2014-08-13 13:35:04]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/halcanary@google.com/464333002/1

[commit-bot: I haz the power, 2014-08-13 13:35:05]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from a valid reviewer till 2014-08-13 19:35 UTC

[hal.canary, 2014-08-13 13:36:19]

The CQ bit was checked by halcanary@google.com

[commit-bot: I haz the power, 2014-08-13 13:37:04]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/halcanary@google.com/464333002/20001

[commit-bot: I haz the power, 2014-08-13 13:37:06]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from a valid reviewer till 2014-08-13 19:37 UTC

[mtklein, 2014-08-13 14:13:30]

lgtm

[commit-bot: I haz the power, 2014-08-13 14:14:41]

Committed patchset #1 (20001) as bc677c801523034fd68261e843db597c6625a8bc