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.h

@@ -250,6 +250,13 @@ class HTMLInputElement final : public TextControlElement,
       const nsAString& aValue) override;
   virtual bool HasCachedSelection() override;
 
+  /**
+   * TextEditorValueEquals() is designed for internal use so that aValue
+   * shouldn't include \r character.  It should be handled before calling this
+   * with nsContentUtils::PlatformToDOMLineBreaks().
+   */
+  bool TextEditorValueEquals(const nsAString& aValue) const;
+
   // Methods for nsFormFillController so it can do selection operations on input
   // types the HTML spec doesn't support them on, like "email".
   uint32_t GetSelectionStartIgnoringType(ErrorResult& aRv);