Ensure that request ids from redirects get a different id

Bug: 
Change-Id: I95354a0bdb4e700ec2fc9e8a2d8d079e5f153ab2
Reviewed-on: https://chromium-review.googlesource.com/662081
Commit-Queue: David Vallet <dvallet@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: David Vallet <dvallet@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501224}

headless/public/util/generic_url_request_job.cc

@@ -189,7 +189,8 @@ void GenericURLRequestJob::GetLoadTimingInfo(
 }
 
 uint64_t GenericURLRequestJob::GenericURLRequestJob::GetRequestId() const {
-  return request_->identifier();
+  return request_->identifier() +
+         (static_cast<uint64_t>(request_->url_chain().size()) << 32);
 }
 
 const net::URLRequest* GenericURLRequestJob::GetURLRequest() const {