Implement base::PowerMonitor::IsOnBatteryPower() for OSX.

base/power_monitor/power_monitor_device_source.h

Index: base/power_monitor/power_monitor_device_source.h
diff --git a/base/power_monitor/power_monitor_device_source.h b/base/power_monitor/power_monitor_device_source.h
index 2dabac8865ed49c81d98e34f6b58a920667d962e..69237cd643413d14b2d2f14a099607d29bd94063 100644
--- a/base/power_monitor/power_monitor_device_source.h
+++ b/base/power_monitor/power_monitor_device_source.h
@@ -15,18 +15,8 @@
 
 #if defined(OS_WIN)
 #include <windows.h>
-
-// Windows HiRes timers drain the battery faster so we need to know the battery
-// status.  This isn't true for other platforms.
-#define ENABLE_BATTERY_MONITORING 1
-#else
-#undef ENABLE_BATTERY_MONITORING
 #endif  // !OS_WIN
 
-#if defined(ENABLE_BATTERY_MONITORING)
-#include "base/timer/timer.h"
-#endif  // defined(ENABLE_BATTERY_MONITORING)
-
 #if defined(OS_IOS)
 #include <objc/runtime.h>
 #endif  // OS_IOS
@@ -93,19 +83,11 @@ class BASE_EXPORT PowerMonitorDeviceSource : public PowerMonitorSource {
   // false otherwise.
   bool IsOnBatteryPowerImpl() override;

[Nico, 2016/09/27 20:59:33]

Ah, because this is in the vtable. But what references the vtable?

...probably this
https://cs.chromium.org/chromium/src/ios/web/app/web_main_loop.mm?rcl=0&l=75

So now we're in a situation where the function isn't implemented, but we have a
stub. I guess Rohit figured it's not super important, so landing this as-is
seems fine, but we should probably delete that ref (not in this Cl, and not you)
and then the ios implementation of the empty function, so that if someone
actually wants to use it they'll get a linker error informing them they need to
implement it.

 
-  // Checks the battery status and notifies observers if the battery
-  // status has changed.
-  void BatteryCheck();
-
 #if defined(OS_IOS)
   // Holds pointers to system event notification observers.
   std::vector<id> notification_observers_;
 #endif
 
-#if defined(ENABLE_BATTERY_MONITORING)
-  base::OneShotTimer delayed_battery_check_;
-#endif
-
 #if defined(OS_WIN)
   PowerMessageWindow power_message_window_;
 #endif