Add offscreen mode to visualbench

Replaces "nvpr mode" with "offscreen mode" and a flag to enable/
disable nvpr. Assigns new config names to the various combinations of
flags and begins printing them with the bench results in order to match
nanobench.

BUG=skia:

Review URL: https://codereview.chromium.org/1473253002
diff --git a/tools/VisualBench/WrappedBenchmark.h b/tools/VisualBench/WrappedBenchmark.h
index dba6134..d16556c 100644
--- a/tools/VisualBench/WrappedBenchmark.h
+++ b/tools/VisualBench/WrappedBenchmark.h
@@ -95,10 +95,10 @@
 };
 
 // Create an MSAA & NVPR-enabled GPU backend
-class NvprWrappedBenchmark : public WrappedBenchmark {
+class GpuWrappedBenchmark : public WrappedBenchmark {
 public:
-    explicit NvprWrappedBenchmark(const SkSurfaceProps& surfaceProps, Benchmark* bench,
-                                  int numSamples)
+    explicit GpuWrappedBenchmark(const SkSurfaceProps& surfaceProps, Benchmark* bench,
+                                 int numSamples)
         : INHERITED(surfaceProps, bench)
         , fNumSamples(numSamples) {}