Subset decoding benchmarks

It was my goal to create benchmarks that could measure all
of the use cases that we have identified.  I think single
subsets, translating, and scaling are the important ones.

It might be a good idea to discuss the document in greater
detail as well.  I just wanted to share this to aid the
discussion.
https://docs.google.com/a/google.com/document/d/1OxW96GDMAlw6dnzNXmiNX-F9oDBBlGXzSsgd0DMIkbI/edit?usp=sharing

BUG=skia:

Review URL: https://codereview.chromium.org/1160953002
13 files changed