code: function restoreFontSize() {
          var fsStyle      = prefs.get("fontSize"),
              fsAdjustment = PreferencesManager.getViewState("fontSizeAdjustment");
  
          if (fsAdjustment) {
              // Always remove the old view state even if we also have the new view state.
              PreferencesManager.setViewState("fontSizeAdjustment");
  
              if (!fsStyle) {
                  // Migrate the old view state to the new one.
                  fsStyle = (DEFAULT_FONT_SIZE + fsAdjustment) + "px";
                  prefs.set("fontSize", fsStyle);
              }
          }
  
          if (fsStyle) {
              _removeDynamicFontSize();
              _addDynamicFontSize(fsStyle);
          }
      }
docs: Restores the font size using the saved style and migrates the old fontSizeAdjustment
  view state to the new fontSize, when required