[headless] Add default browser context to HeadlessBrowser.

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 021e91064fbc4e099945b7cc782d1b430afc5a7a..c3043f006ad40216b6313cac3bfbd416898492e1 100644
--- a/headless/public/headless_browser.h
+++ b/headless/public/headless_browser.h
@@ -56,6 +56,12 @@ class HEADLESS_EXPORT HeadlessBrowser {
   virtual HeadlessBrowserContext* GetBrowserContextForId(
       const std::string& id) = 0;
 
+  // Allows setting and getting the browser context that DevTools will create
+  // new targets in by default.
+  virtual void SetDefaultBrowserContext(
+      HeadlessBrowserContext* browser_context) = 0;
+  virtual HeadlessBrowserContext* GetDefaultBrowserContext() = 0;
+
   // Returns a task runner for submitting work to the browser file thread.
   virtual scoped_refptr<base::SingleThreadTaskRunner> BrowserFileThread()
       const = 0;