Fix visualbench flicker on triple buffered platforms

BUG=skia:

Review URL: https://codereview.chromium.org/1441073005
diff --git a/tools/VisualBench/VisualStreamTimingModule.cpp b/tools/VisualBench/VisualStreamTimingModule.cpp
index cb748ca..0ac698e 100644
--- a/tools/VisualBench/VisualStreamTimingModule.cpp
+++ b/tools/VisualBench/VisualStreamTimingModule.cpp
@@ -25,7 +25,7 @@
         case kReset_InitState:
             // This will flicker unfortunately, but as we are reseting the GLContext each bench,
             // we unfortunately don't have a choice
-            fOwner->clear(canvas, SK_ColorWHITE, 2);
+            fOwner->clear(canvas, SK_ColorWHITE, 3);
             fBenchmarkStream->current()->preTimingHooks(canvas);
             break;
         case kNone_InitState: