Bug 1698786: part 6) Add some `const`-correctness to `mozInlineSpellWordUtil`. r=smaug

Depends on D109038

Differential Revision: https://phabricator.services.mozilla.com/D109269

extensions/spellcheck/src/mozInlineSpellWordUtil.cpp

@@ -1012,9 +1012,8 @@ NodeOffset mozInlineSpellWordUtil::MapSoftTextOffsetToDOMPosition(
   return NodeOffset(nullptr, -1);
 }
 
-int32_t mozInlineSpellWordUtil::FindRealWordContaining(int32_t aSoftTextOffset,
-                                                       DOMMapHint aHint,
-                                                       bool aSearchForward) {
+int32_t mozInlineSpellWordUtil::FindRealWordContaining(
+    int32_t aSoftTextOffset, DOMMapHint aHint, bool aSearchForward) const {
   NS_ASSERTION(mSoftTextValid,
                "Soft text must be valid if we're to map out of it");
   if (!mSoftTextValid) return -1;

extensions/spellcheck/src/mozInlineSpellWordUtil.h

@@ -194,7 +194,7 @@ class MOZ_STACK_CLASS mozInlineSpellWordUtil {
   // position, search forward until we do find a word and return that (if
   // found).
   int32_t FindRealWordContaining(int32_t aSoftTextOffset, DOMMapHint aHint,
-                                 bool aSearchForward);
+                                 bool aSearchForward) const;
 
   // build mSoftText and mSoftTextDOMMapping
   void BuildSoftText();