headless: Implement screenshot capturing

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 95332ac14ecec153ca8fbd8807301ca117ddd2ed..737a1f7e6103f25d3fdf601071e340a777e695ae 100644
--- a/headless/lib/browser/headless_browser_impl.h
+++ b/headless/lib/browser/headless_browser_impl.h
@@ -9,6 +9,7 @@
 
 #include <memory>
 #include <unordered_map>
+#include <vector>
 
 #include "base/synchronization/lock.h"
 #include "headless/lib/browser/headless_web_contents_impl.h"
@@ -38,6 +39,8 @@ class HeadlessBrowserImpl : public HeadlessBrowser {
                                          const gfx::Size& size) override;
   scoped_refptr<base::SingleThreadTaskRunner> BrowserMainThread()
       const override;
+  scoped_refptr<base::SingleThreadTaskRunner> BrowserFileThread()
+      const override;
 
   void Shutdown() override;
 
@@ -73,6 +76,7 @@ class HeadlessBrowserImpl : public HeadlessBrowser {
   std::unordered_map<HeadlessWebContents*, std::unique_ptr<HeadlessWebContents>>
       web_contents_;
 
+ private:
   DISALLOW_COPY_AND_ASSIGN(HeadlessBrowserImpl);
 };