extend SkBenchmark to allow a bench to return a durationScale, which allows it to perform fewer act…

bench/benchmain.cpp

Index: bench/benchmain.cpp
diff --git a/bench/benchmain.cpp b/bench/benchmain.cpp
index 4d385a23e63060d2409dfc8d16603d25705f96b0..7ad8a6850868b6a3f4fc88911ba0d3a77fd7a296 100644
--- a/bench/benchmain.cpp
+++ b/bench/benchmain.cpp
@@ -915,7 +915,7 @@ int tool_main(int argc, char** argv) {
                         SkPicture::kUsePathBoundsForClip_RecordingFlag);
                 }
 
-                timer->start();
+                timer->start(bench->getDurationScale());
                 if (NULL != canvas) {
                     canvas->save();
                 }