Bug 1800553 - part 2: Change HTMLAnchorElement::GetText to a const method r=smaug

This needs to change `nsContentUtils` methods because of used by the method.
Then, we can make some there callers are also `const` methods.

Depends on D162065

Differential Revision: https://phabricator.services.mozilla.com/D162066

dom/html/HTMLTextAreaElement.cpp

@@ -343,7 +343,7 @@ void HTMLTextAreaElement::SetLastValueChangeWasInteractive(
 }
 
 void HTMLTextAreaElement::GetDefaultValue(nsAString& aDefaultValue,
-                                          ErrorResult& aError) {
+                                          ErrorResult& aError) const {
   if (!nsContentUtils::GetNodeTextContent(this, false, aDefaultValue,
                                           fallible)) {
     aError.Throw(NS_ERROR_OUT_OF_MEMORY);