At some point, several releases ago, the icon went low rez and has remained that way.
Created attachment 201649 [details] Screen snap
Nothing has changed about the icon. If there's a bug here, it's outside of WebKit's control.
Outside of WebKit / WebCore and more of OS layer (based on reference attached PDF) and too old that it might not be applicable anymore. Since it was never confirmed. Marking this as "RESOLVED WONTFIX".