Warmup for one extra frame in nanobench

On ANGLE, at least, this frame gives us much more consistent results.

Bug: skia:
Change-Id: Ifdecc8451ef51490c08057645214738180b1a366
Reviewed-on: https://skia-review.googlesource.com/57884
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

bench/nanobench.cpp

@@ -397,7 +397,7 @@ static int setup_gpu_bench(Target* target, Benchmark* bench, int maxGpuFrameLag)
 
     // Pretty much the same deal as the calibration: do some warmup to make
     // sure we're timing steady-state pipelined frames.
-    for (int i = 0; i < maxGpuFrameLag - 1; i++) {
+    for (int i = 0; i < maxGpuFrameLag; i++) {
         time(loops, bench, target);
     }