Adding the 'filter' context attribute to 2D canvas

LayoutTests/fast/canvas/canvas-filter-global-alpha.html

Index: LayoutTests/fast/canvas/canvas-filter-global-alpha.html
diff --git a/LayoutTests/fast/canvas/canvas-filter-global-alpha.html b/LayoutTests/fast/canvas/canvas-filter-global-alpha.html
new file mode 100644
index 0000000000000000000000000000000000000000..a2e48fb742ea1cf87fdda75ce3b38d9fe88f5fc7
--- /dev/null
+++ b/LayoutTests/fast/canvas/canvas-filter-global-alpha.html
@@ -0,0 +1,9 @@
+<canvas id="canvas" width="100" height="100"></canvas>
+<script>
+var canvas = document.getElementById('canvas');
+var ctx = canvas.getContext('2d');
+ctx.filter = 'drop-shadow(0px 20px black)';
+ctx.globalAlpha = 0.5;
+ctx.fillStyle = '#0f0';
+ctx.fillRect(25, 25, 50, 20);
+</script>