Bug 1700051: part 12) Declare `mozInlineSpellStatus::mSpellChecker` `const`. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D109747

extensions/spellcheck/src/mozInlineSpellChecker.h

@@ -60,7 +60,7 @@ class mozInlineSpellStatus {
   // Return true if we plan to spell-check everything
   bool IsFullSpellCheck() const { return mOp == eOpChange && !mRange; }
 
-  RefPtr<mozInlineSpellChecker> mSpellChecker;
+  const RefPtr<mozInlineSpellChecker> mSpellChecker;
 
   enum Operation {
     eOpChange,        // for SpellCheckAfterEditorChange except