[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
w3c/clipboard-apis#195
I2S: https://groups.google.com/a/chromium.org/g/blink-dev/c/pjpN9Lwv5Tk/m/KrAZRbdwAQAJ?utm_medium=email&utm_source=footer&pli=1

Bug: 1483026

Change-Id: Ief7c0786833548d2fb51215cefbc39e5930af875

clipboard-apis/clipboard-item.https.html

@@ -95,4 +95,18 @@
   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_false(ClipboardItem.supports('web '));
+  assert_false(ClipboardItem.supports('web')); // without space.
+  assert_false(ClipboardItem.supports('web foo'));
+  assert_false(ClipboardItem.supports('foo/bar'));
+  assert_true(ClipboardItem.supports('web foo/bar'));
+  assert_true(ClipboardItem.supports('web text/html'));
+  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>