[skia_benchmarking_extension] fix off-by-one timing reporting.

skia/ext/benchmarking_canvas.cc

Index: skia/ext/benchmarking_canvas.cc
diff --git a/skia/ext/benchmarking_canvas.cc b/skia/ext/benchmarking_canvas.cc
index 60395158cb2096148951ca0c46704d14f7eb162e..d83252b4c553a7f734ff29c8758fc22dcac43bf7 100644
--- a/skia/ext/benchmarking_canvas.cc
+++ b/skia/ext/benchmarking_canvas.cc
@@ -203,7 +203,8 @@ AutoStamper::AutoStamper(TimingCanvas *timing_canvas)
 
 AutoStamper::~AutoStamper() {
   base::TimeDelta delta = base::TimeTicks::HighResNow() - start_ticks_;
-  int command_index = timing_canvas_->tracking_canvas_->CommandCount();
+  int command_index = timing_canvas_->tracking_canvas_->CommandCount() - 1;
+  DCHECK_GE(command_index, 0);
   timing_canvas_->timings_map_[command_index] = delta;
 }