Bug 1597679 - part 3: Create methods to compare given string with values of TextControlState, nsTextControlFrame, HTMLInputElement and HTMLTextAreaElement r=smaug

For avoiding unnecessary copy of string buffer only for comparing setting
value and current value, especially with `nsAutoString`, this patch
creates `*Equals()` methods for every class.

And also this avoids to call `nsContentUtils::PlatformToDOMLineBreaks()` in
most paths.

Differential Revision: https://phabricator.services.mozilla.com/D54331

dom/html/HTMLInputElement.cpp

@@ -6756,6 +6756,11 @@ void HTMLInputElement::GetTextEditorValue(nsAString& aValue,
   }
 }
 
+bool HTMLInputElement::TextEditorValueEquals(const nsAString& aValue) const {
+  TextControlState* state = GetEditorState();
+  return state ? state->ValueEquals(aValue) : aValue.IsEmpty();
+}
+
 void HTMLInputElement::InitializeKeyboardEventListeners() {
   TextControlState* state = GetEditorState();
   if (state) {