[fuchsia] Add NetworkChangeNotifier singleton creation logic.

The NetworkChangeNotifier is ready to use, now let's actually use it.
:)

TEST=Exercised change event detection on Ethernet w/Acer Switch 12.

Bug: 843443
Change-Id: I23c4ebbcb536dba55ee6b5b86ecdef775f90bd2f
Reviewed-on: https://chromium-review.googlesource.com/c/1257548Reviewed-by: Nick Harper <nharper@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596026}

net/base/network_change_notifier.cc

@@ -33,6 +33,8 @@
 #include "net/base/network_change_notifier_linux.h"
 #elif defined(OS_MACOSX)
 #include "net/base/network_change_notifier_mac.h"
+#elif defined(OS_FUCHSIA)
+#include "net/base/network_change_notifier_fuchsia.h"
 #endif
 
 namespace net {
@@ -217,6 +219,8 @@ NetworkChangeNotifier* NetworkChangeNotifier::Create() {
   return new NetworkChangeNotifierLinux(std::unordered_set<std::string>());
 #elif defined(OS_MACOSX)
   return new NetworkChangeNotifierMac();
+#elif defined(OS_FUCHSIA)
+  return new NetworkChangeNotifierFuchsia();
 #else
   NOTIMPLEMENTED();
   return NULL;