Adds NetworkChangeNotifier::GetCurrentMaxBandwidth.

This is a step in supporting the max bandwidth attribute of NetInfo V3.  http://w3c.github.io/netinfo/

There is a default implementation so that each platform can add support independently.

BUG=412741

Review URL: https://codereview.chromium.org/556253003

Cr-Commit-Position: refs/heads/master@{#298268}
diff --git a/net/net.gypi b/net/net.gypi
index 35ec25e..8ed69df 100644
--- a/net/net.gypi
+++ b/net/net.gypi
@@ -1274,6 +1274,7 @@
       'base/net_log_unittest.h',
       'base/net_util_unittest.cc',
       'base/net_util_icu_unittest.cc',
+      'base/network_change_notifier_unittest.cc',
       'base/network_change_notifier_win_unittest.cc',
       'base/prioritized_dispatcher_unittest.cc',
       'base/priority_queue_unittest.cc',