Support header modifications in WebRequestProxyingURLLoaderFactory

This change adds support for handling the |to_be_removed_request_headers|
and |modified_request_headers| parameters to FollowRedirect(). The changes
are reflected in the stored request object and passed on to the underlying
URLLoader.

Bug: 880069
Change-Id: I5c79b358537c741467bd5e2d3a71d19cfa5dec42
Reviewed-on: https://chromium-review.googlesource.com/1207712Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589001}

extensions/browser/api/web_request/web_request_proxying_url_loader_factory.cc

@@ -121,12 +121,19 @@ void WebRequestProxyingURLLoaderFactory::InProgressRequest::FollowRedirect(
     const base::Optional<std::vector<std::string>>&
         to_be_removed_request_headers,
     const base::Optional<net::HttpRequestHeaders>& modified_request_headers) {
-  DCHECK(!modified_request_headers.has_value()) << "Redirect with modified "
-                                                   "headers was not supported "
-                                                   "yet. crbug.com/845683";
+  if (to_be_removed_request_headers) {
+    for (const std::string& header : *to_be_removed_request_headers)
+      request_.headers.RemoveHeader(header);
+  }
+
+  if (modified_request_headers)
+    request_.headers.MergeFrom(*modified_request_headers);
+
+  if (target_loader_.is_bound()) {
+    target_loader_->FollowRedirect(to_be_removed_request_headers,
+                                   modified_request_headers);
+  }
 
-  if (target_loader_.is_bound())
-    target_loader_->FollowRedirect(base::nullopt, base::nullopt);
   Restart();
 }