add bench for fontcache performance (measurement)

add bench for fontcache performance (measurement)

BUG=

Committed: https://code.google.com/p/skia/source/detail?r=9508

[reed1, 2013-06-11 19:18:49]

Committed patchset #1 manually as r9508 (presubmit successful).

[tomhudson, 2013-06-12 01:48:11]

This will be a nice approach for simple changes to the font cache, but my worry
with some of the font cache proposals is that they'll have inconsistent
performance - we hit a cache purge and all of a sudden have to regenerate every
glyph. I don't think that would show up clearly in aggregate like this, and we'd
need something that runs for a long time, is known to trigger cache overflows,
and looks at max time rather than average time?

[reed1, 2013-06-12 13:41:14]

Do you think this is a bad micro-bench, or that we should write others as well?