[headless] Expose DevToolsTarget via HeadlessBrowser + add tracing test.

headless/lib/browser/headless_browser_impl.h

Index: headless/lib/browser/headless_browser_impl.h
diff --git a/headless/lib/browser/headless_browser_impl.h b/headless/lib/browser/headless_browser_impl.h
index 8f67953f92c9d9f8b386b5a8e5cf5d7a6264823d..b92fb759a3f5ba6f91f9ed5f414e3bc301903240 100644
--- a/headless/lib/browser/headless_browser_impl.h
+++ b/headless/lib/browser/headless_browser_impl.h
@@ -24,7 +24,8 @@ class HeadlessBrowserContextImpl;
 class HeadlessBrowserMainParts;
 
 // Exported for tests.
-class HEADLESS_EXPORT HeadlessBrowserImpl : public HeadlessBrowser {
+class HEADLESS_EXPORT HeadlessBrowserImpl : public HeadlessBrowser,
+                                            public HeadlessDevToolsTarget {
  public:
   HeadlessBrowserImpl(
       const base::Callback<void(HeadlessBrowser*)>& on_start_callback,
@@ -49,6 +50,13 @@ class HEADLESS_EXPORT HeadlessBrowserImpl : public HeadlessBrowser {
   void SetDefaultBrowserContext(
       HeadlessBrowserContext* browser_context) override;
   HeadlessBrowserContext* GetDefaultBrowserContext() override;
+  HeadlessDevToolsTarget* GetDevToolsTarget() override;
+
+  // HeadlessDevToolsTarget implementation:
+  bool AttachClient(HeadlessDevToolsClient* client) override;
+  void ForceAttachClient(HeadlessDevToolsClient* client) override;
+  void DetachClient(HeadlessDevToolsClient* client) override;
+  bool IsAttached() override;
 
   void set_browser_main_parts(HeadlessBrowserMainParts* browser_main_parts);
   HeadlessBrowserMainParts* browser_main_parts() const;
@@ -82,6 +90,8 @@ class HEADLESS_EXPORT HeadlessBrowserImpl : public HeadlessBrowser {
       browser_contexts_;
   HeadlessBrowserContext* default_browser_context_;  // Not owned.
 
+  scoped_refptr<content::DevToolsAgentHost> agent_host_;
+
   base::WeakPtrFactory<HeadlessBrowserImpl> weak_ptr_factory_;
 
  private: