Adds an iOS implementation of base::SystemMonitor.

base/base.gypi

Index: base/base.gypi
diff --git a/base/base.gypi b/base/base.gypi
index 8f6650962ac19b8e17163d9b2bb9fac8d32b1b9d..3e8f5c97baef9ec5840a57e7bcf18f7af9f1f9b3 100644
--- a/base/base.gypi
+++ b/base/base.gypi
@@ -348,6 +348,7 @@
           'system_monitor/system_monitor.cc',
           'system_monitor/system_monitor.h',
           'system_monitor/system_monitor_android.cc',
+          'system_monitor/system_monitor_ios.mm',
           'system_monitor/system_monitor_mac.mm',
           'system_monitor/system_monitor_posix.cc',
           'system_monitor/system_monitor_win.cc',
@@ -573,7 +574,6 @@
               ['exclude', '^json'],
               ['exclude', '^message_pump'],
               ['exclude', '^process_util'],
-              ['exclude', '^system_monitor'],
               ['exclude', '^values'],
             ],
           }],