Instead of always doing the newline fixup on the `<textarea>`
placeholder, only do it when the placeholder attribute itself changes.
This avoids doing string replacements when they are not necessary (such
as when a `<textarea>` switches from having text to only having the
placeholder and viceversa).
In addition make it so that `DOMString::clear` preserves the string
allocation when called.
Testing: This is just a small optimization so testing should not be
necessary. Existing behavior is verified via WPT tests.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>