### Preflight Checklist
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* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
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### Problem Description
[powerMonitor](https://www.electronjs.org/docs/api/power-monitor) triggers events when the computer goes on AC or on battery. But there's no way to know on app startup whether the system is already on battery or AC. An event isn't triggered until the state change happens.

I learned from [this comment](https://github.com/electron/electron/issues/18457#issuecomment-496628006) that Electron inherits powerMonitor from Chromium. Well, Chromium's `PowerMonitor` class has a [IsOnBatteryPower](https://chromium.googlesource.com/chromium/src/+/master/base/power_monitor/power_monitor.h#52) method.

### Proposed Solution
Add an `isOnBatteryPower` method to the [powerMonitor](https://www.electronjs.org/docs/api/power-monitor) module.

### Alternatives Considered
Alternatively, Electron could fire the `on-ac` or `on-battery` event on app startup. In fact, perhaps this should also be the case. (I verified it does not currently do this)

### Additional Information
n/a
shell/browser/api/electron_api_power_monitor.cc
const ui::IdleState& in) {
    switch (in) {
      case ui::IDLE_STATE_ACTIVE:
        return StringToV8(isolate, "active");
      case ui::IDLE_STATE_IDLE:
        return StringToV8(isolate, "idle");
      case ui::IDLE_STATE_LOCKED:
        return StringToV8(isolate, "locked");
      case ui::IDLE_STATE_UNKNOWN:
      default:
        return StringToV8(isolate, "unknown");
    }
  }
};
}  
namespace electron {
namespace api {
gin::WrapperInfo PowerMonitor::kWrapperInfo = {gin::kEmbedderNativeGin};
PowerMonitor::PowerMonitor(v8::Isolate* isolate) {
#if defined(OS_MAC)
  Browser::Get()->SetShutdownHandler(base::BindRepeating(
      &PowerMonitor::ShouldShutdown, base::Unretained(this)));
#endif
  base::PowerMonitor::AddObserver(this);
#if defined(OS_MAC) || defined(OS_WIN)
  InitPlatformSpecificMonitors();
#endif
}
PowerMonitor::~PowerMonitor() {