Several users of the content/ public API have been trying to match NavigationHandles with the URLRequest that corresponds to the navigation. The current interface has some edge cases due to MIME sniffing and ordering of events between the UI and IO thread. I propose introducing a new method, WebContentsObserver::DidCreateURLRequestForNavigation(NavigationHandle*) to make it easy for content/ embedders to do so.

See the rationale in this doc https://docs.google.com/a/chromium.org/document/d/1FqxC_mgBEi5N1tVCxgj3M7P2yrRTOdxZdShzEpxdknM/edit?usp=sharing.