Resolve a few memory leaks in tests.

Resolve a few memory leaks in tests.

Purge the global scaled image cache after use in tests.  The cache was
right to hold on to the pixels indefinitely, but this change makes it
easier to run down actual memory leaks.

Add SK_DECLARE_INST_COUNT to several classes.

BUG=skia:

Committed: http://code.google.com/p/skia/source/detail?r=13171

[hal.canary, 2014-01-23 19:05:13]

ptal

[reed1, 2014-01-24 17:17:44]

lgtm

[commit-bot: I haz the power, 2014-01-24 18:21:18]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/halcanary@google.com/145443004/1

[commit-bot: I haz the power, 2014-01-24 18:33:26]

Change committed as 13171