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

headless/lib/browser/headless_web_contents_impl.h

Index: headless/lib/browser/headless_web_contents_impl.h
diff --git a/headless/lib/browser/headless_web_contents_impl.h b/headless/lib/browser/headless_web_contents_impl.h
index c966fca3601dbe6ac1d5bc3525b093f04931094d..10fba499458a1a48267d7dbb83f89961d0f0708e 100644
--- a/headless/lib/browser/headless_web_contents_impl.h
+++ b/headless/lib/browser/headless_web_contents_impl.h
@@ -12,6 +12,7 @@
 
 #include "content/public/browser/render_process_host_observer.h"
 #include "content/public/browser/web_contents_observer.h"
+#include "headless/lib/browser/headless_window_tree_host.h"
 #include "headless/public/headless_devtools_target.h"
 #include "headless/public/headless_export.h"
 #include "headless/public/headless_web_contents.h"
@@ -78,6 +79,13 @@ class HEADLESS_EXPORT HeadlessWebContentsImpl
   HeadlessBrowserImpl* browser() const;
   HeadlessBrowserContextImpl* browser_context() const;
 
+  void set_window_tree_host(std::unique_ptr<HeadlessWindowTreeHost> host) {
+    window_tree_host_ = std::move(host);
+  }
+  HeadlessWindowTreeHost* window_tree_host() const {
+    return window_tree_host_.get();
+  }
+
  private:
   // Takes ownership of |web_contents|.
   HeadlessWebContentsImpl(content::WebContents* web_contents,
@@ -89,6 +97,7 @@ class HEADLESS_EXPORT HeadlessWebContentsImpl
 
   class Delegate;
   std::unique_ptr<Delegate> web_contents_delegate_;
+  std::unique_ptr<HeadlessWindowTreeHost> window_tree_host_;
   std::unique_ptr<content::WebContents> web_contents_;
   scoped_refptr<content::DevToolsAgentHost> agent_host_;
   std::list<MojoService> mojo_services_;