Make BrowserView DragRegionViews children of the WebContents view

Previously they were children of the `InspectableWebContentsView` view,
which caused this assertion to fail:

f993888424/brightray/browser/mac/bry_inspectable_web_contents_view.mm (L162)

atom/browser/native_browser_view.h

@@ -9,6 +9,7 @@
 
 #include "atom/common/draggable_region.h"
 #include "base/macros.h"
+#include "content/public/browser/web_contents.h"
 #include "third_party/skia/include/core/SkColor.h"
 
 namespace brightray {
@@ -39,6 +40,7 @@ class NativeBrowserView {
   }
 
   brightray::InspectableWebContentsView* GetInspectableWebContentsView();
+  content::WebContents* GetWebContents();
 
   virtual void SetAutoResizeFlags(uint8_t flags) = 0;
   virtual void SetBounds(const gfx::Rect& bounds) = 0;