[InputEvent] Allow transpose and yank in plain-text field

third_party/WebKit/LayoutTests/fast/events/inputevents/inputevent-transpose.html

 <title>InputEvent: macOS Transpose</title>
 <script src="../../../resources/testharness.js"></script>
 <script src="../../../resources/testharnessreport.js"></script>
 <div id="editable" contenteditable></div>
+<textarea id="txt"></textarea>
 <script>
 test(() => {
     assert_not_equals(window.testRunner, undefined, 'This test requires testRunner.');
 
     let eventRecorder = '';
     document.addEventListener('beforeinput', event => {
         eventRecorder += `beforeinput-${event.inputType}-${event.data}-`;
     });
     document.addEventListener('input', event => {
         eventRecorder += `input-${event.inputType}`;
     });
 
     const editable = document.getElementById('editable');
     editable.innerHTML = 'abc';
     editable.focus();
     const selection = window.getSelection();
     selection.collapse(editable, 1); // End of first line.
 
     // Test Transpose.
     eventRecorder = '';
     testRunner.execCommand('transpose');
     assert_equals(editable.innerHTML, 'acb');
     assert_equals(eventRecorder, 'beforeinput-insertTranspose-cb-input-insertTranspose');
-});
+}, 'Transpose on contenteditable');
+
+test(() => {
+    assert_not_equals(window.testRunner, undefined, 'This test requires testRunner.');
+
+    let eventRecorder = '';
+    document.addEventListener('beforeinput', event => {
+        eventRecorder += `beforeinput-${event.inputType}-${event.data}-`;
+    });
+    document.addEventListener('input', event => {
+        eventRecorder += `input-${event.inputType}`;
+    });
+
+    const txt = document.getElementById('txt');
+    txt.value = 'abc';
+    txt.focus();
+    txt.setSelectionRange(3, 3); // End of first line.
+
+    // Test Transpose.
+    eventRecorder = '';
+    testRunner.execCommand('transpose');
+    assert_equals(txt.value, 'acb');
+    assert_equals(eventRecorder, 'beforeinput-insertTranspose-cb-input-insertTranspose');
+}, 'Transpose on <textarea>');
 </script>