Fix bugs in sheriff-o-matic's pixel zoomer.

Tools/GardeningServer/ui/ct-results-comparison-zoomer.html

 <!--
 Copyright 2014 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <polymer-element name="ct-results-comparison-zoomer" attributes="expectedUrl actualUrl diffUrl position">
   <template>
     <style>
       :host {
         background-color: silver;
         border: 1px solid gray;
         box-shadow: 0 0 5px rgba(0, 0, 0, 0.75);
         display: flex;
         pointer-events: none;
         position: fixed;
         top: 0;
         width: 100%;
+        z-index: 1;
       }
 
       :host > * {
         flex: 1;
       }
 
       .label {
         background-color: white;
         text-align: center;
       }
@@ skipping 63 matching lines @@
 
         this._draw(this.$.expectedZoomer);
         this._draw(this.$.actualZoomer);
         this._draw(this.$.diffZoomer);
       },
 
       _draw: function(imageContainer) {
         var canvas = imageContainer.querySelector('canvas');
         canvas.width = imageContainer.clientWidth;
         canvas.height = imageContainer.clientHeight;
+        this._drawCanvas(canvas.getContext('2d'), imageContainer);
+      },
 
-        var ctx = canvas.getContext('2d');
-        ctx.imageSmoothingEnabled = false;
-        ctx.translate(imageContainer.clientWidth / 2, imageContainer.clientHeight / 2);
-        ctx.translate(-this.position.y * kZoomedResultWidth, -this.position.x * kZoomedResultHeight);
-        ctx.strokeRect(-1.5, -1.5, kZoomedResultWidth + 2, kZoomedResultHeight + 2);
-        ctx.scale(kZoomFactor, kZoomFactor);
-        ctx.drawImage(imageContainer.querySelector('img'), 0, 0);
+      _drawCanvas: function(context, imageContainer) {
+        context.imageSmoothingEnabled = false;
+        context.translate(imageContainer.clientWidth / 2, imageContainer.clientHeight / 2);
+        context.translate(-this.position.x * kZoomedResultWidth, -this.position.y * kZoomedResultHeight);
+        context.strokeRect(-1.5, -1.5, kZoomedResultWidth + 2, kZoomedResultHeight + 2);
+        context.scale(kZoomFactor, kZoomFactor);
+        context.drawImage(imageContainer.querySelector('img'), 0, 0);
       },
     });
   })();
   </script>
 </polymer-element>