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

headless/lib/browser/headless_web_contents_impl.cc

 // Copyright 2015 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_web_contents_impl.h"
 
 #include <string>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 141 matching lines @@
     content::WebContents* web_contents,
     HeadlessBrowserContextImpl* browser_context) {
   std::unique_ptr<HeadlessWebContentsImpl> headless_web_contents =
       base::WrapUnique(
           new HeadlessWebContentsImpl(web_contents, browser_context));
 
   return headless_web_contents;
 }
 
 void HeadlessWebContentsImpl::InitializeScreen(const gfx::Size& initial_size) {
-  browser()->PlatformInitializeWebContents(initial_size, web_contents_.get());
+  browser()->PlatformInitializeWebContents(initial_size, this);
 }
 
 HeadlessWebContentsImpl::HeadlessWebContentsImpl(
     content::WebContents* web_contents,
     HeadlessBrowserContextImpl* browser_context)
     : content::WebContentsObserver(web_contents),
       web_contents_delegate_(
           new HeadlessWebContentsImpl::Delegate(browser_context)),
       web_contents_(web_contents),
       agent_host_(content::DevToolsAgentHost::GetOrCreateFor(web_contents)),
@@ skipping 146 matching lines @@
 HeadlessWebContents::Builder::MojoService::MojoService() {}
 
 HeadlessWebContents::Builder::MojoService::MojoService(
     const std::string& service_name,
     const base::Callback<void(mojo::ScopedMessagePipeHandle)>& service_factory)
     : service_name(service_name), service_factory(service_factory) {}
 
 HeadlessWebContents::Builder::MojoService::~MojoService() {}
 
 }  // namespace headless