Make OffscreenCanvas 2D context support globalAlpha

third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-2d-globalAlpha.html

+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<script>
+test(function() {
+    function assertPixelApprox(ctx, x, y, r, g, b, a, pos, colour, tolerance) {
+        var imgdata = ctx.getImageData(x, y, 1, 1);
+        var diff = Math.max(Math.abs(r-imgdata.data[0]), Math.abs(g-imgdata.data[1]), Math.abs(b-imgdata.data[2]), Math.abs(a-imgdata.data[3]));
+        assert_less_than_equal(diff, tolerance);
+    }
+    var offscreenCanvas = new OffscreenCanvas(100, 100);
+    var ctx = offscreenCanvas.getContext('2d');
+    ctx.fillStyle = '#0f0';
+    ctx.fillRect(0, 0, 100, 100);
+    ctx.globalAlpha = 0.01; // avoid any potential alpha = 0 optimisations.

[xlai (Olivia), 2016/11/23 17:26:43]

Could you use a more obvious number like 0.2, 0.7 etc for globalAlpha? Also,
include more globalAlpha values.

Also, could you use a reference test?

+    ctx.beginPath();
+    ctx.fillStyle = '#f00';
+    ctx.rect(0, 0, 100, 100);
+    ctx.fill();
+    assertPixelApprox(ctx, 50, 25, 2, 253, 0, 255, "50,25", "2,253,0,255", 2);
+});
+</script>