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/VisualInteractiveModule.cpp b/tools/VisualBench/VisualInteractiveModule.cpp
index b597b05..73bc53e 100755
--- a/tools/VisualBench/VisualInteractiveModule.cpp
+++ b/tools/VisualBench/VisualInteractiveModule.cpp
@@ -15,7 +15,7 @@
 __SK_FORCE_IMAGE_DECODER_LINKING;
 
 VisualInteractiveModule::VisualInteractiveModule(VisualBench* owner)
-    : INHERITED(owner, false)
+    : INHERITED(owner)
     , fCurrentMeasurement(0)
     , fAdvance(false) {
     memset(fMeasurements, 0, sizeof(fMeasurements));