Use event init dictionaries in core/{css,html}

Source/core/html/canvas/WebGLContextEventInit.idl

Index: Source/core/html/canvas/WebGLContextEventInit.idl
diff --git a/Source/core/html/canvas/WebGLContextEventInit.idl b/Source/core/html/canvas/WebGLContextEventInit.idl
new file mode 100644
index 0000000000000000000000000000000000000000..7a7ced807733f64ded39c7bd1a0f8cc0f83e687e
--- /dev/null
+++ b/Source/core/html/canvas/WebGLContextEventInit.idl
@@ -0,0 +1,9 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// https://www.khronos.org/registry/webgl/specs/1.0/#5.15
+
+dictionary WebGLContextEventInit : EventInit {
+    DOMString statusMessage;
+};