Implement PowerMonitorDeviceSource::IsOnBatteryPowerImpl on iOS

The NOTIMPLEMENTED is reached at startup.
Implement the method.

Bug: 852369
Change-Id: I843f7cd7e168a0b4317e030e3f48349ef3cd025c
Reviewed-on: https://chromium-review.googlesource.com/1098965Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567588}

base/power_monitor/power_monitor_device_source_ios.mm

@@ -9,8 +9,17 @@
 namespace base {
 
 bool PowerMonitorDeviceSource::IsOnBatteryPowerImpl() {
-  NOTIMPLEMENTED();
+#if TARGET_IPHONE_SIMULATOR
   return false;
+#else
+  UIDevice* currentDevice = [UIDevice currentDevice];
+  BOOL isCurrentAppMonitoringBattery = currentDevice.isBatteryMonitoringEnabled;
+  [UIDevice currentDevice].batteryMonitoringEnabled = YES;
+  UIDeviceBatteryState batteryState = [UIDevice currentDevice].batteryState;
+  currentDevice.batteryMonitoringEnabled = isCurrentAppMonitoringBattery;
+  DCHECK(batteryState != UIDeviceBatteryStateUnknown);
+  return batteryState == UIDeviceBatteryStateUnplugged;
+#endif
 }
 
 void PowerMonitorDeviceSource::PlatformInit() {