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
diff --git a/gyp/bench.gypi b/gyp/bench.gypi
index f0a203e..a637202 100644
--- a/gyp/bench.gypi
+++ b/gyp/bench.gypi
@@ -4,6 +4,8 @@
 # found in the LICENSE file.
 {
   'include_dirs': [
+    '../bench/subset',
+    '../bench',
     '../src/core',
     '../src/effects',
     '../src/gpu',