fix(rrweb): Guard cross-origin Element read in ShadowDomManager.obs…

[s1gr1d]

Closes #321

For a cross-origin iframe, contentWindow does not throw — the browser returns a truthy restricted proxy. So the !iframeWindow guard in observeAttachShadow passes, and the bare iframeWindow.Element read is the first real cross-origin access. It throws a SecurityError, or yields undefined and patchAttachShadow then throws a TypeError on element.prototype. Both escape into the host page's window.onerror.

Fixed by reading Element inside try/catch and bailing out when it is
unreachable, since replay cannot observe a frame the browser blocks.

[s1gr1d]

Do we need this file? 🤔

[chargome]

l: we can likely delete this