Adding the 'filter' context attribute to 2D canvas

LayoutTests/fast/canvas/canvas-filter-units-em.html

Index: LayoutTests/fast/canvas/canvas-filter-units-em.html
diff --git a/LayoutTests/fast/canvas/canvas-filter-units-em.html b/LayoutTests/fast/canvas/canvas-filter-units-em.html
new file mode 100644
index 0000000000000000000000000000000000000000..3355c62eab52339dbb8b17ad3b3f85aac6eb043d
--- /dev/null
+++ b/LayoutTests/fast/canvas/canvas-filter-units-em.html
@@ -0,0 +1,13 @@
+<canvas id="canvas" width="100" height="100"></canvas>
+<script>
+var canvas = document.getElementById('canvas');
+var ctx = canvas.getContext('2d');
+ctx.font = '20px sans-serif';
+ctx.filter = 'drop-shadow(0em .5em black)';
+ctx.fillStyle = '#0f0';
+ctx.fillRect(25, 25, 25, 40);
+canvas.style.fontSize = '5px';
+ctx.font = '4em sans-serif';
+ctx.filter = 'drop-shadow(0em .5em black)';
+ctx.fillRect(50, 25, 25, 40);
+</script>