Minor cleanup in nanobench

bench/nanobench.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef nanobench_DEFINED
 #define nanobench_DEFINED
 
@@ skipping 29 matching lines @@
     explicit Target(const Config& c) : config(c) { }
     virtual ~Target() { }
 
     const Config config;
     SkAutoTDelete<SkSurface> surface;
 
     /** Called once per target, immediately before any timing or drawing. */
     virtual void setup() { }
 
     /** Called *after* the clock timer is started, before the benchmark
-        is drawn.  */
+        is drawn. Most back ends just return the canvas passed in,
+        but some may replace it. */
     virtual SkCanvas* beginTiming(SkCanvas* canvas) { return canvas; }
 
     /** Called *after* a benchmark is drawn, but before the clock timer
         is stopped.  */
     virtual void endTiming() { }
 
     /** Called between benchmarks (or between calibration and measured
         runs) to make sure all pending work in drivers / threads is
         complete. */
     virtual void fence() { }
 
     /** CPU-like targets can just be timed, but GPU-like
         targets need to pay attention to frame boundaries
         or other similar details. */
     virtual bool needsFrameTiming() const { return false; }
 
     /** Called once per target, during program initialization.
         Returns false if initialization fails. */
     virtual bool init(SkImageInfo info, Benchmark* bench);
 
     /** Stores any pixels drawn to the screen in the bitmap.
         Returns false on error. */
     virtual bool capturePixels(SkBitmap* bmp);
 
     /** Writes any config-specific data to the log. */
     virtual void fillOptions(ResultsWriter*) { }
+
+    SkCanvas* getCanvas() const {
+        if (!surface.get()) {
+            return NULL;
+        }
+        return surface->getCanvas();
+    }
 };
 
 #endif  // nanobench_DEFINED