Adding the 'filter' context attribute to 2D canvas

LayoutTests/fast/canvas/canvas-filter-multiple-drop-shadows.html

Index: LayoutTests/fast/canvas/canvas-filter-multiple-drop-shadows.html
diff --git a/LayoutTests/fast/canvas/canvas-filter-multiple-drop-shadows.html b/LayoutTests/fast/canvas/canvas-filter-multiple-drop-shadows.html
new file mode 100644
index 0000000000000000000000000000000000000000..476c2238eda746ff76dbed753de43aca2fd53ae8
--- /dev/null
+++ b/LayoutTests/fast/canvas/canvas-filter-multiple-drop-shadows.html
@@ -0,0 +1,8 @@
+<canvas id="canvas" width="100" height="100"></canvas>
+<script>
+var canvas = document.getElementById('canvas');
+var ctx = canvas.getContext('2d');
+ctx.fillStyle = '#0f0';
+ctx.filter = 'drop-shadow(0px 10px black) drop-shadow(10px 0px #ccc)';
+ctx.fillRect(20, 25, 50, 40);
+</script>