Add visualbench option to not reset between samples

Adds a --reset flag that controls whether we teardown the graphics
context between samples. Also modifies the timing algorithm to always/
only pre-warm after resetting the context.

BUG=skia:

Review URL: https://codereview.chromium.org/1442643007
diff --git a/tools/VisualBench/VisualStreamTimingModule.h b/tools/VisualBench/VisualStreamTimingModule.h
index 5dbec69..80c67e8 100644
--- a/tools/VisualBench/VisualStreamTimingModule.h
+++ b/tools/VisualBench/VisualStreamTimingModule.h
@@ -24,7 +24,7 @@
  */
 class VisualStreamTimingModule : public VisualModule {
 public:
-    VisualStreamTimingModule(VisualBench* owner, bool preWarmBeforeSample);
+    VisualStreamTimingModule(VisualBench* owner);
     void draw(SkCanvas* canvas) override;
 
 private:
@@ -44,7 +44,6 @@
         kNewBenchmark_InitState,
     };
     InitState fInitState;
-    bool fPreWarmBeforeSample;
 
     // support framework
     VisualBench* fOwner;