The frameNamesToWindow map was a holdover from the BrowserWindowProxy
IPC shim. Since nativeWindowOpen became the only code path, Blink's
FrameTree::FindOrCreateFrameForNavigation resolves named window targets
directly in the renderer, scoped to the opener's browsing context
group. When a matching named window exists, Blink navigates it without
ever sending a CreateNewWindow IPC to the browser, so this map was
never consulted in the legitimate same-opener case.

The only time the map found a match was when two unrelated renderers
happened to use the same target name, in which case openGuestWindow
would short-circuit before consuming the guest WebContents that
Chromium had already created for the new window, leaking it.

Adds a test verifying Blink handles same-opener named-target reuse
end-to-end without any browser-side tracking.

Co-authored-by: Sam Attard <sattard@anthropic.com>