Add BASE_API to SystemMonitor::PowerObserver to be able to

base/system_monitor/system_monitor.h

Index: base/system_monitor/system_monitor.h
===================================================================
--- base/system_monitor/system_monitor.h	(revision 86456)
+++ base/system_monitor/system_monitor.h	(working copy)
@@ -66,7 +66,7 @@
   // are finished. Observers should implement quick callback functions; if
   // lengthy operations are needed, the observer should take care to invoke
   // the operation on an appropriate thread.
-  class PowerObserver {
+  class BASE_API PowerObserver {
    public:
     // Notification of a change in power status of the computer, such
     // as from switching between battery and A/C power.