Rename image async attribute to decoding attribute.

This patch renames the async attribute to be the decoding attribute
due to discussion on whatwg/html#1920

This also adds wpt test for the decoding attribute

[email protected], [email protected]

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I0524b7e3849c9a922f358c0592d34c30a7fd8914
Reviewed-on: https://chromium-review.googlesource.com/770106
Commit-Queue: vmpstr <[email protected]>
Reviewed-by: Chris Harrelson <[email protected]>
Cr-Commit-Position: refs/heads/master@{#519453}

html/dom/elements-embedded.js

@@ -11,6 +11,7 @@ var embeddedElements = {
     width: {type: "unsigned long", customGetter: true},
     height: {type: "unsigned long", customGetter: true},
     referrerPolicy: {type: "enum", keywords: ["", "no-referrer", "no-referrer-when-downgrade", "same-origin", "origin", "strict-origin", "origin-when-cross-origin", "strict-origin-when-cross-origin", "unsafe-url"]},
+    decoding: {type: "enum", keywords: ["async", "sync", "auto"], defaultVal: "auto", invalidVal: "auto"},
 
     // Obsolete
     name: "string",

html/dom/reflection.js

@@ -669,8 +669,12 @@ ReflectionTests.reflects = function(data, idlName, idlObj, domName, domObj) {
             }
 
             if (data.keywords[i].length > 1) {
-                domTests.push(data.keywords[i].slice(1));
-                idlTests.push(data.keywords[i].slice(1));
+                var sliced = data.keywords[i].slice(1);
+                // If slicing a value yields another valid value, then skip it since it results in duplicate tests.
+                if (data.keywords.indexOf(sliced) == -1) {
+                    domTests.push(sliced);
+                    idlTests.push(sliced);
+                }
             }
 
             if (data.keywords[i] != data.keywords[i].toLowerCase()) {