Save/Restore around bench runs.

BUG=skia:

Review URL: https://codereview.chromium.org/619353002
diff --git a/bench/Benchmark.cpp b/bench/Benchmark.cpp
index 9205ba9..88e385d 100644
--- a/bench/Benchmark.cpp
+++ b/bench/Benchmark.cpp
@@ -7,6 +7,7 @@
 
 #include "Benchmark.h"
 
+#include "SkCanvas.h"
 #include "SkPaint.h"
 #include "SkParse.h"
 
@@ -37,6 +38,7 @@
 }
 
 void Benchmark::draw(const int loops, SkCanvas* canvas) {
+    SkAutoCanvasRestore ar(canvas, true/*save now*/);
     this->onDraw(loops, canvas);
 }