title: Electron "-webkit-app-region: drag" doesn't work in BrowserView content
body: <p>In my Electron app, I'm creating a <code>BrowserWindow</code>:</p>
  
  <pre><code>let window = new BrowserView({
      title: "My App",
      frame: false,
      show: false
  }
  </code></pre>
  
  <p>Then I'm loading a <code>BrowserView</code>:</p>
  
  <pre><code>let view = new BrowserView();
  window.setBrowserView(view);
  view.setBounds({ x: 0, y: 0, width: window.getContentBounds().width, height: window.getContentBounds().height });
  view.setAutoResize({ width: true, height: true});
  view.webContents.loadURL("MY URL HERE");
  </code></pre>
  
  <p>In the content that is loaded into the <code>BrowserView</code>, I'm trying to create a custom title bar, and I've done something like:</p>
  
  <pre><code>&lt;div id="windowBar"&gt;
      &lt;div id="draggableArea"&gt;
          &lt;div id="title"&gt;&lt;span&gt;My App&lt;/span&gt;&lt;/div&gt;
          &lt;div id="controls"&gt;
              &lt;!-- The window controls to minimize/maximize/close the window here !--&gt;
          &lt;/div&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>I'd like to allow it to drag the entire Electron window, so I've added the CSS:</p>
  
  <pre><code>#windowBar {
      height: 30px;
      width: 100%;
      position: fixed;
  }
  
  #draggableArea {
      width: 100%;
      height: 100%;
      -webkit-app-region: drag;
  }
  
  #title {
      -webkit-app-region: drag;
      user-select: none;
  }
  
  #controls {
      -webkit-app-region: no-drag;
  }
  </code></pre>
  
  <p>I can't tell from the docs, and from everything else I've found online, if this is supposed to be working. Or does the title bar's HTML need to be loaded directly into the <code>BrowserWindow</code> for this to work?</p>