headless: Implement screenshot capturing

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 c1f5b4d0974d9bf7d7ca58c63ef9fb34b7695f33..b869fc44fdf2c48fcfd5d870b8eb6ce2c760e3de 100644
--- a/headless/public/headless_browser.h
+++ b/headless/public/headless_browser.h
@@ -8,6 +8,7 @@
 #include <memory>
 #include <string>
 #include <unordered_map>
+#include <vector>
 
 #include "base/callback.h"
 #include "base/macros.h"
@@ -51,6 +52,10 @@ class HEADLESS_EXPORT HeadlessBrowser {
   virtual scoped_refptr<base::SingleThreadTaskRunner> BrowserMainThread()
       const = 0;
 
+  // Returns a task runner for submitting work to the browser file thread.
+  virtual scoped_refptr<base::SingleThreadTaskRunner> BrowserFileThread()
+      const = 0;
+
   // Requests browser to stop as soon as possible. |Run| will return as soon as
   // browser stops.
   virtual void Shutdown() = 0;