Remove get/set currentPath in canvas

LayoutTests/fast/canvas/script-tests/canvas-currentPath-crash.js

-description("Series of tests to ensure that currentPath does not take non-Path values.");
-var ctx = document.createElement('canvas').getContext('2d');
-ctx.fillStyle = "green";
-ctx.beginPath();
-ctx.rect(0,0,100,100);
-
-ctx.currentPath = null;
-ctx.currentPath = 0;
-ctx.currentPath = {};
-ctx.currentPath = new Array();
-ctx.currentPath = undefined;
-
-ctx.fill();
-var imageData = ctx.getImageData(50, 50, 1, 1);
-var imgdata = imageData.data;
-shouldBe("imgdata[0]", "0");
-shouldBe("imgdata[1]", "128");
-shouldBe("imgdata[2]", "0");