Android HWUI backend Nanobench

Uses filtering canvas from utils/android, shared with DM.
Follow-up plans in https://skbug.com/3589, https://skbug.com/3595

R=djsollen@google.com

Review URL: https://codereview.chromium.org/1029423010
diff --git a/bench/Benchmark.h b/bench/Benchmark.h
index c30167e..5ba6e91 100644
--- a/bench/Benchmark.h
+++ b/bench/Benchmark.h
@@ -57,6 +57,7 @@
         kRaster_Backend,
         kGPU_Backend,
         kPDF_Backend,
+        kHWUI_Backend,
     };
 
     // Call to determine whether the benchmark is intended for