Have nanobench pay attention to --threads.

TBR=

BUG=skia:

Review URL: https://codereview.chromium.org/1229953002
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index a2f349b..f10fd63 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -935,7 +935,7 @@
 int nanobench_main() {
     SetupCrashHandler();
     SkAutoGraphics ag;
-    SkTaskGroup::Enabler enabled;
+    SkTaskGroup::Enabler enabled(FLAGS_threads);
 
 #if SK_SUPPORT_GPU
     GrContextOptions grContextOpts;