Add a function to add extra headers from NavigationThrottle

content/browser/loader/navigation_url_loader.h

Index: content/browser/loader/navigation_url_loader.h
diff --git a/content/browser/loader/navigation_url_loader.h b/content/browser/loader/navigation_url_loader.h
index d3d58663da257b82d1373d979584cc83f0714077..9b954cb975a1243d230d38cd1d22f0acee81cec4 100644
--- a/content/browser/loader/navigation_url_loader.h
+++ b/content/browser/loader/navigation_url_loader.h
@@ -5,6 +5,8 @@
 #ifndef CONTENT_BROWSER_LOADER_NAVIGATION_URL_LOADER_H_
 #define CONTENT_BROWSER_LOADER_NAVIGATION_URL_LOADER_H_
 
+#include <vector>
+
 #include "base/basictypes.h"
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
@@ -46,7 +48,9 @@ class CONTENT_EXPORT NavigationURLLoader {
 
   // Called in response to OnRequestRedirected to continue processing the
   // request.

[davidben, 2015/12/03 23:44:57]

Document what extra_headers does.

NB: This API is kind of weird because the headers actually stick from one
redirect to the next.

-  virtual void FollowRedirect() = 0;
+  virtual void FollowRedirect(
+      const std::vector<std::pair<std::string, std::string> >&

[davidben, 2015/12/03 23:44:57]

 > > can be >>

+          extra_headers) = 0;
 
  protected:
   NavigationURLLoader() {}