[headless] Use individual aura::WindowTreeHosts per WebContents.

headless/lib/browser/headless_browser_impl_mac.mm

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "headless/lib/browser/headless_browser_impl.h"
 
 #include "content/public/browser/web_contents.h"
 #include "ui/base/cocoa/base_view.h"
 
 namespace headless {
 
 void HeadlessBrowserImpl::PlatformInitialize() {}
 
 void HeadlessBrowserImpl::PlatformCreateWindow() {}
 
 void HeadlessBrowserImpl::PlatformInitializeWebContents(
     const gfx::Size& initial_size,
-    content::WebContents* web_contents) {
-  NSView* web_view = web_contents->GetNativeView();
+    HeadlessWebContentsImpl* web_contents) {
+  NSView* web_view = web_contents->web_contents()->GetNativeView();
   [web_view setAutoresizingMask:(NSViewWidthSizable | NSViewHeightSizable)];
 
   NSRect frame = NSMakeRect(0, 0, initial_size.width(), initial_size.height());
   [web_view setFrame:frame];
   return;
 }
 
 }  // namespace headless