Add accessor for can_wake_from_suspend_ in AlarmTimer

Consumers of this class (powerd) might want to know if it actually can
wake the system from suspend so that they can try alternatives if it
doesn't have this ability.

BUG=chrome-os-partner:31970
TBR=derat

Review URL: https://codereview.chromium.org/685503006

Cr-Commit-Position: refs/heads/master@{#302158}

components/timers/alarm_timer.h

@@ -61,6 +61,8 @@ class AlarmTimer : public base::Timer,
 
   ~AlarmTimer() override;
 
+  bool can_wake_from_suspend() { return can_wake_from_suspend_; }
+
   // Timer overrides.
   void Stop() override;
   void Reset() override;