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Setup() is called on each test and it will set the global
NetworkChangeNotifierFactory multiple times, causing a CHECK
failure in NetworkChangeNotifier::SetFactory().

This CL fixes the issue by not setting the factory if it is
already set.

BUG=1023050

Change-Id: I61e2ca89e0c2409bdd8f8725c2e96bf706618da8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1909810Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714606}