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### Issue Details

* **Electron Version:**
  * 6.0.0-beta.15
* **Operating System:**
  * Windows 10 (1903)
* **Last Known Working Electron version:**
  * 5.0.8

### Expected Behaviour
When a frameless window is maximized, and then enters full screen from that state, it should fill the entire screen with no border.

### Actual Behaviour
If a frameless window enters fullscreen from a maximized state, a ~10px white border appears all around the screen.
(Hard to show in a screenshot on a white page…)

### To Reproduce
Fiddle gist: https://gist.github.com/caesar/12c97746e9d8b0e9bd2ee7e4958c63ad

### Screenshots
It's hard to see the issue here because the border is white and so is the background of this page… but if you look carefully, the second time I the window enters fullscreen (when it does it from maximized), a white border appears.
![Kapture 2019-07-26 at 23 28 21](https://user-images.githubusercontent.com/1566516/61984625-20991200-affd-11e9-9e8a-9b4faed1fea8.gif)

shell/browser/ui/views/root_view.cc
#include "shell/browser/ui/views/root_view.h"
#include "content/public/browser/native_web_keyboard_event.h"
#include "shell/browser/native_window.h"
#include "shell/browser/ui/views/menu_bar.h"
namespace electron {
namespace {
#if defined(OS_WIN)
const int kMenuBarHeight = 20;
#else
const int kMenuBarHeight = 25;
#endif
bool IsAltKey(const content::NativeWebKeyboardEvent& event) {
  return event.windows_key_code == ui::VKEY_MENU;
}
bool IsAltModifier(const content::NativeWebKeyboardEvent& event) {
  typedef content::NativeWebKeyboardEvent::Modifiers Modifiers;
  int modifiers = event.GetModifiers();
  modifiers &= ~Modifiers::kNumLockOn;
  modifiers &= ~Modifiers::kCapsLockOn;
  return (modifiers == Modifiers::kAltKey) ||
         (modifiers == (Modifiers::kAltKey | Modifiers::kIsLeft)) ||
         (modifiers == (Modifiers::kAltKey | Modifiers::kIsRight));
}
}  
RootView::RootView(NativeWindow* window)