Add MultiPictureDraw to nanobench

I would like some guard against performance regressions on our side before turning layer hoisting on in Chromium.

TBR=bsalomon@google.com

Committed: https://skia.googlesource.com/skia/+/0ddad31012dabfc1267effc8071d37f7d606efbe

Review URL: https://codereview.chromium.org/731973005
diff --git a/bench/Benchmark.cpp b/bench/Benchmark.cpp
index 88e385d..6040f91 100644
--- a/bench/Benchmark.cpp
+++ b/bench/Benchmark.cpp
@@ -37,6 +37,14 @@
     this->onPreDraw();
 }
 
+void Benchmark::perCanvasPreDraw(SkCanvas* canvas) {
+    this->onPerCanvasPreDraw(canvas);
+}
+
+void Benchmark::perCanvasPostDraw(SkCanvas* canvas) {
+    this->onPerCanvasPostDraw(canvas);
+}
+
 void Benchmark::draw(const int loops, SkCanvas* canvas) {
     SkAutoCanvasRestore ar(canvas, true/*save now*/);
     this->onDraw(loops, canvas);