Correctly record new initial window size after auto-shrinking

This call to _recordWindowSize() fails because this is not RFPHelper there.

Consequences:

1. Console noise (TypeError: this._recordWindowSize is not a function) as reported by @pierov
2. If the user resizes the window to its actual original size (whose height was off by the extra chrome, e.g. from the ephemeral bookmarks bar), it gets unexpectedly shrunk down again to proper rounded size, which might be confusing.