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

headless/public/headless_browser.h

Index: headless/public/headless_browser.h
diff --git a/headless/public/headless_browser.h b/headless/public/headless_browser.h
index 4b18f565b9e8fc2f2b8cb91ad0bb885a26a8a64e..473125a49637586ea54abe1f60dd2edacacd32e6 100644
--- a/headless/public/headless_browser.h
+++ b/headless/public/headless_browser.h
@@ -46,6 +46,12 @@ class HEADLESS_EXPORT HeadlessBrowser {
 
   virtual std::vector<HeadlessBrowserContext*> GetAllBrowserContexts() = 0;
 
+  // Return a DevTools target corresponding to this tab. Note that this method

[Sami, 2017/04/26 15:47:06]

s/tab/browser/?

[Eric Seckler, 2017/04/27 09:14:20]

Uh, indeed! :)

+  // only returns a valid target after browser has been initialized on the main
+  // thread. The target only supports the domains available on the
+  // browser endpoint excluding the Tethering domain.
+  virtual HeadlessDevToolsTarget* GetDevToolsTarget() = 0;
+
   // Returns the HeadlessWebContents associated with the
   // |devtools_agent_host_id| if any.  Otherwise returns null.
   virtual HeadlessWebContents* GetWebContentsForDevToolsAgentHostId(