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/VisualLightweightBenchModule.cpp b/tools/VisualBench/VisualLightweightBenchModule.cpp
index af939ff..baca168 100644
--- a/tools/VisualBench/VisualLightweightBenchModule.cpp
+++ b/tools/VisualBench/VisualLightweightBenchModule.cpp
@@ -42,7 +42,7 @@
 #define HUMANIZE(time) humanize(time).c_str()
 
 VisualLightweightBenchModule::VisualLightweightBenchModule(VisualBench* owner)
-    : INHERITED(owner, true)
+    : INHERITED(owner)
     , fCurrentSample(0)
     , fResults(new ResultsWriter) {
     // Print header