Bug 1461452 - Clear the cached emoji font-list pref in response to a pref change or font-list rebuild. r=m_kato

gfx/thebes/gfxPlatformFontList.cpp

@@ -1721,6 +1721,7 @@ gfxPlatformFontList::ClearLangGroupPrefFonts()
         }
     }
     mCJKPrefLangs.Clear();
+    mEmojiPrefFont = nullptr;
 }
 
 // Support for memory reporting