DM+VB: WallTimer -> SkTime::GetNSecs().

The same timer with a simpler interface.

BUG=skia:

Review URL: https://codereview.chromium.org/1427033003
diff --git a/tools/VisualBench/TimingStateMachine.h b/tools/VisualBench/TimingStateMachine.h
index 3f40b6d..322c333 100644
--- a/tools/VisualBench/TimingStateMachine.h
+++ b/tools/VisualBench/TimingStateMachine.h
@@ -67,7 +67,7 @@
     int fCurrentFrame;
     int fLoops;
     double fLastMeasurement;
-    WallTimer fTimer;
+    double fStartTime;
     State fState;
     InnerState fInnerState;
 };