Modify StartGloballyReachableCheck to take a ClientSocketFactory.

This change switches from always using a DefaultClientSocketFactory to the socket factory from a RequestImpl's ResolveContext. This is necessary for brokering to function properly, as StartGloballyReachableCheck needs to be able to use a BrokeredClientSocketFactory to actually receive a brokered UdpClientSocket.

Not every unit test suite currently creates a HostResolverManager using the UrlRequestContextBuilder, so this change does keep usage of DefaultClientSocketFactory if a RequestImpl's ResolveContext is null.

Bug: 1295329

Change-Id: I7cc7064bb97989353cc809a3c21abb1aab58fb93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4537784


Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Liza Burakova <liza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149101}