Canvas element that use the alpha attribute don't display correctly.

LayoutTests/fast/canvas/alpha.js

 description("Series of tests for canvas alpha");
 
+if (window.testRunner)
+    testRunner.dumpAsTextWithPixelResults();
+
 var canvas1 = document.getElementById("canvas1");
 var canvas2 = document.getElementById("canvas2");
 var canvas3 = document.getElementById("canvas3");
 var canvas4 = document.getElementById("canvas4");
 
 var ctx1 = canvas1.getContext("2d");
 var ctx2 = canvas2.getContext("2d", {} );
 var ctx3 = canvas3.getContext("2d", { alpha: false } );
 var ctx4 = canvas4.getContext("2d", { alpha: true } );
 
 shouldBe("ctx1.getContextAttributes().alpha", "true");
 var imgData1 = ctx1.getImageData(0, 0, 1, 1);
 shouldBe("imgData1.data[0]", "0");
 shouldBe("imgData1.data[1]", "0");
 shouldBe("imgData1.data[2]", "0");
 shouldBe("imgData1.data[3]", "0");
 
 shouldBe("ctx2.getContextAttributes().alpha", "true");
 var imgData2 = ctx2.getImageData(0, 0, 1, 1);
 shouldBe("imgData2.data[0]", "0");
 shouldBe("imgData2.data[1]", "0");
 shouldBe("imgData2.data[2]", "0");
 shouldBe("imgData2.data[3]", "0");
 
 shouldBe("ctx3.getContextAttributes().alpha", "false");
-var imgData3 = ctx3.getImageData(0, 0, 1, 1);
-shouldBe("imgData3.data[0]", "0");
-shouldBe("imgData3.data[1]", "0");
-shouldBe("imgData3.data[2]", "0");
-shouldBe("imgData3.data[3]", "255");
 
 shouldBe("ctx4.getContextAttributes().alpha", "true");
 var imgData4 = ctx4.getImageData(0, 0, 1, 1);
 shouldBe("imgData4.data[0]", "0");
 shouldBe("imgData4.data[1]", "0");
 shouldBe("imgData4.data[2]", "0");
 shouldBe("imgData4.data[3]", "0");
 
 // Check that mutating the returned value of getContextAttributes() doesn't
 // affect the existing canvas, or the values of subsequent calls to
 // getContextAttributes().
 var attrs = ctx4.getContextAttributes();
 shouldBe("attrs.alpha", "true");
 attrs.alpha = false;
 var imgData4 = ctx4.getImageData(0, 0, 1, 1);
 shouldBe("ctx4.getContextAttributes().alpha", "true");
 shouldBe("imgData4.data[0]", "0");
 shouldBe("imgData4.data[1]", "0");
 shouldBe("imgData4.data[2]", "0");
 shouldBe("imgData4.data[3]", "0");