[Async Clipboard API] Add supports method to async clipboard API.

In this change we are adding a new static method `supports` to the
ClipboardItem interface to help web authors detect clipboard format
types that are supported by Chromium.
Github Issue: w3c/clipboard-apis#170
Spec: w3c/clipboard-apis#192

Bug: 1483026
Change-Id: Ief7c0786833548d2fb51215cefbc39e5930af875

clipboard-apis/clipboard-item.https.html

@@ -95,4 +95,13 @@
   const text = await (new Response(blob)).text();
   assert_equals(text, 'xxx');
 }, "getType(DOMString invalid type) converts DOMString to Blob");
+
+promise_test(async () => {
+  assert_true(ClipboardItem.supports('text/plain'));
+  assert_true(ClipboardItem.supports('text/html'));
+  assert_true(ClipboardItem.supports('image/png'));
+  assert_true(ClipboardItem.supports('web '));
+  assert_false(ClipboardItem.supports('image/svg+xml'));
+  assert_false(ClipboardItem.supports('not a/real type'));
+}, "supports(DOMString) returns true for types that are supported, false otherwise");
 </script>