fix[devtools]: still show overlay, if getClientRects is not implemented

[hoxyq]

Follow-up to https://github.com/facebook/react/pull/34653.

React Native doesn't implement getClientRect, since this is applicable to CSS box, which is not a concept for Native (maybe yet).

I am loosening the condition that gates showOverlay() call to pass if getClientRect is not implemented.

Conceptually, everything that is inside react-devtools-shared/backend should be Host-agnostic, because both on Web and Native it is installed inside the Host JavaScript runtime, be it main frame of the page, or RN instance. Since overlay & highlighting logic also lives there, it should also follow these principles.