Add deduping capability to SkPathHeap

Once Chromium starts holding on to paths and we can actually reuse cached path data (e.g., masks & geometry) we will need a way to preserve that reuse in the skps. This CL begins adding that capability. More analysis & profiling needs to be done before it is always enabled.

When enabled it does make the disabled path de-duping test in the Canvas unit test pass.

BUG=skia:507
Committed: http://code.google.com/p/skia/source/detail?r=13709

[robertphillips, 2014-03-07 15:30:34]

Adding Mike K. in case he would like to chime in.

[mtklein, 2014-03-07 17:43:50]

This implementation LGTM.  Again, I'd probably have used a hash table here
instead of binary search, but seems fine to start with whatever works and worry
about speed only when it matters.

At some point I'd like to talk about why we keep writing first drafts using
not-hashtables for table-ish things.  If I had to guess, SkTDynamicHash has an
intimidating name and complicated interface?  I would be happy to look at making
it more usable.

[robertphillips, 2014-03-07 18:11:28]

The CQ bit was checked by robertphillips@google.com

[commit-bot: I haz the power, 2014-03-07 18:11:39]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/robertphillips@google.com/190923002/1

[commit-bot: I haz the power, 2014-03-07 18:42:28]

Change committed as 13709