Clear the canvas before drawing the gears.

BUG=skia:
R=robertphillips@google.com

Author: jcgregorio@google.com

Review URL: https://codereview.chromium.org/141113002

git-svn-id: http://skia.googlecode.com/svn/trunk@13114 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/experimental/SkV8Example/gears.js b/experimental/SkV8Example/gears.js
index 7eb4c5b..5ba2f99 100644
--- a/experimental/SkV8Example/gears.js
+++ b/experimental/SkV8Example/gears.js
@@ -97,6 +97,9 @@
   function draw(ctx) {
     ctx.resetTransform();
 
+    ctx.fillStyle = "#FFFFFF";
+    ctx.fillRect(0, 0, 499, 499);
+
     rotation += DeltaTheta;
     if (rotation >= Math.PI*2) {
       rotation = 0;