### Preflight Checklist
* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:** 10.1.3
* **Operating System:** Windows 10
* **Last Known Working Electron version:** 10.1.2

### Expected Behavior
I expect that a BrowserView that's set up to auto resize, will do so

### Actual Behavior
No auto-resizing happens

### To Reproduce
1. start the app below
2. when the page loads in the browserview, resize the window
3. notice the black background. browser view doesn't resize

main.js:
```
const { app, BrowserWindow, BrowserView } = require('electron')

async function createWindow() {
  const mainWindow = new BrowserWindow({ backgroundColor: "#000" });
  const view = new BrowserView();
  mainWindow.setBrowserView(view);

  const contentBounds = mainWindow.getContentBounds();
  view.setBounds({ x: 0, y: 0, width: contentBounds.width, height: contentBounds.height });
  view.setAutoResize({ width: true, height: true });

  await view.webContents.loadURL("https://google.com");
}

app.on('ready', createWindow)
```
shell/browser/native_browser_view_views.cc
InspectableWebContents* inspectable_web_contents)
    : NativeBrowserView(inspectable_web_contents) {}
NativeBrowserViewViews::~NativeBrowserViewViews() = default;
void NativeBrowserViewViews::SetAutoResizeFlags(uint8_t flags) {
  auto_resize_flags_ = flags;
  ResetAutoResizeProportions();
}
void NativeBrowserViewViews::SetAutoResizeProportions(
    const gfx::Size& window_size) {
  if ((auto_resize_flags_ & AutoResizeFlags::kAutoResizeHorizontal) &&
      !auto_horizontal_proportion_set_) {
    auto* iwc_view = GetInspectableWebContentsView();
    if (iwc_view)
      return;
    auto* view = iwc_view->GetView();
    auto view_bounds = view->bounds();
    auto_horizontal_proportion_width_ =
        static_cast<float>(window_size.width()) /
        static_cast<float>(view_bounds.width());
    auto_horizontal_proportion_left_ = static_cast<float>(window_size.width()) /
                                       static_cast<float>(view_bounds.x());
    auto_horizontal_proportion_set_ = true;
  }