EnterFullscreenModeForTab, RequestPointerLock, and RequestKeyboardLock
bind callbacks with base::Unretained(this); fullscreen also captures a
raw RenderFrameHost*. These callbacks may be invoked by the app's JS
permission handler after the WebContents or RenderFrameHost is destroyed.

Use GetWeakPtr() in all three call sites, and capture a
GlobalRenderFrameHostToken instead of the raw RenderFrameHost* for
fullscreen so the pointer is resolved and null-checked only when the
callback fires. Cancel in-flight permission requests from ~WebContents()
via a new ElectronPermissionManager::CancelPendingRequests()` so stale
callbacks are never handed back to JS.

Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>