<textarea>: defaultValue setter should be same as textContent setter.

[1]:
> On setting, it must act as the setter for the element's textContent
> IDL attribute.

The new behavior matches to Firefox and Safari.

This CL is almost equivalent to crrev.com/489300 by Shanmuga Pandi M.
The regression caused by it was fixed by crrev.com/639382 .

[1] https://html.spec.whatwg.org/multipage/form-elements.html#dom-textarea-defaultvalue

Bug: 392843
Change-Id: I8b4ea4cd5fffffdc136ab3812a2c34e43f681345
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538057Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643790}

third_party/blink/renderer/core/html/forms/html_text_area_element.cc

@@ -459,26 +459,7 @@ String HTMLTextAreaElement::defaultValue() const {
 }
 
 void HTMLTextAreaElement::setDefaultValue(const String& default_value) {
-  // To preserve comments, remove only the text nodes, then add a single text
-  // node.
-  HeapVector<Member<Node>> text_nodes;
-  for (Node* n = firstChild(); n; n = n->nextSibling()) {
-    if (n->IsTextNode())
-      text_nodes.push_back(n);
-  }
-  for (const auto& text : text_nodes)
-    RemoveChild(text.Get(), IGNORE_EXCEPTION_FOR_TESTING);
-
-  // Normalize line endings.
-  String value = default_value;
-  value.Replace("\r\n", "\n");
-  value.Replace('\r', '\n');
-
-  InsertBefore(GetDocument().createTextNode(value), firstChild(),
-               IGNORE_EXCEPTION_FOR_TESTING);
-
-  if (!is_dirty_)
-    SetNonDirtyValue(value);
+  setTextContent(default_value);
 }
 
 void HTMLTextAreaElement::SetSuggestedValue(const String& value) {

third_party/blink/web_tests/external/wpt/html/semantics/forms/the-textarea-element/value-defaultValue-textContent-expected.txt

-This is a testharness.js-based test.
-PASS defaultValue and value are the empty string by default
-PASS defaultValue and value are affected by setting textContent
-PASS defaultValue and value are affected by setting nodeValue on a child text node
-PASS defaultValue and value are affected by setting data on a child text node
-PASS defaultValue and value are affected by textContent in combination with appending a text node
-PASS defaultValue and value are affected by textContent in combination with appending a DocumentFragment
-PASS defaultValue and value reflect child text content, not textContent
-FAIL Setting defaultValue wipes out any children, including elements (just like setting textContent) assert_equals: Only one child node should exist expected 1 but got 2
-PASS defaultValue and value treat CRLF differently
-PASS value normalizes CRLF even spread over multiple text nodes
-PASS tests for the value setter
-PASS tests for U+0000 NULL
-Harness: the test ran to completion.
-

third_party/blink/web_tests/fast/forms/textarea/textarea-selection-preservation-expected.txt

@@ -31,8 +31,8 @@ PASS ta.selectionEnd is 7
 PASS ta.selectionStart is 9
 PASS ta.selectionEnd is 9
 - set same defaultValue
-PASS ta.selectionStart is 9
-PASS ta.selectionEnd is 9
+PASS ta.selectionStart is 2
+PASS ta.selectionEnd is 3
 - append a text node
 PASS ta.selectionStart is 12
 PASS ta.selectionEnd is 12

third_party/blink/web_tests/fast/forms/textarea/textarea-selection-preservation.html

@@ -57,8 +57,8 @@
     debug("- set same defaultValue");
     ta.setSelectionRange(2, 3);
     ta.defaultValue = 'abc123456';
-    shouldBe('ta.selectionStart', '9');
-    shouldBe('ta.selectionEnd', '9');
+    shouldBe('ta.selectionStart', '2');
+    shouldBe('ta.selectionEnd', '3');
 
     debug("- append a text node");
     ta.appendChild(document.createTextNode('foo'));