Textarea: Improve typing performance by reducing `value` creation

Every typing in a textarea built the `value` string four times.
  1. UpdateValue() call in SubtreeHasChanged()
  2. IsInnerEditorValueEmpty() call in UpdatePlaceholderVisibility()
  3. InnerEditorValue() call in SubtreeHasChanged()
  4. CachedTextInputInfo::EnsureCached()

We can use Value() for 2, 3 and 4.

- Add TRACE_EVENT0() to HTMLTextAreaElement::SubtreeHasChanged(), which
  can be slow.

- Use ComputeValue() in TextControlElement::InnerEditorValue().
  ComputeValue() is faster than the existing code with StringBuilder.

This CL reduces 5.9ms of typing lag in a textarea with 23k paragraphs.

Bug: 414858401
Change-Id: Ia0abbaf46a1ca67b070b115bb1fac3f1fd97fb5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6549795
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1460519}