Allow fetchers to watch URLRequestContextGetters for context shutdown.
Fetchers outliving the context they're using has been the main cause of
AssertNoURLRequests crashes. This will let the ~26
URLRequestContextGetter implementations be responsible for dealing with
lifetime issues, rather than their 160+ consumers.
URLRequestContextGetter subclasses still need to be modified to actually
inform the fetcher of URLRequestContext shutdown.
BUG=471069
Review URL: https://codereview.chromium.org/
1100603002
Cr-Commit-Position: refs/heads/master@{#329046}