Set _fontSizePrefsLoaded to true before calling _adjustFontSize.

This change fixes the issue #7093 since the flag has to be true in order to set the scroll position to the right location. 

@TomMalbran Can you review this since you're the one who works on this area a lot? 

Note that this specific issue is also introduced by recent CodeMirror update. If I use the CodeMirrror version from sprint-36, then I don't see the issue even if I'm running in the master b64a2a15b with the new view state settings implementation.