Updates on canvas backreference for canvas rendering contexts

third_party/WebKit/LayoutTests/fast/canvas/rendering-contexts-back-references.html

+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<script>
+function make_canvas(type)
+{
+    if (type == "html")
+        return document.createElement("canvas");
+    return new OffscreenCanvas(10, 10);
+}
+
+function test_back_ref(contextType, canvasType)
+{
+    var canvas = make_canvas(canvasType);
+    var ctx = canvas.getContext(contextType);
+    assert_equals(ctx.canvas, canvas, "Back reference to canvas should work.");
+    var anotherCanvas = make_canvas(canvasType);
+    ctx.canvas = anotherCanvas;
+    assert_not_equals(ctx.canvas, anotherCanvas, "Canvas attribute is read only.");
+}
+
+generate_tests(test_back_ref, [
+    ["2d context on html canvas",             "2d",             "html"],
+    ["webgl context on html canvas",          "webgl",          "html"],
+    ["bitmaprenderer context on html canvas", "bitmaprenderer", "html"],
+    ["2d context on offscreen canvas",        "2d",             "offscreen"],
+    ["webgl context on offscreen canvas",     "webgl",          "offscreen"],
+]);
+</script>