Fix null header dereference on CorsURLLoader

Bug: 907073
Change-Id: I86883cc5326ba3a31a588fc1e1e3d7779cbf1df9
Reviewed-on: https://chromium-review.googlesource.com/c/1347960Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610355}

services/network/cors/cors_url_loader.cc

@@ -502,6 +502,8 @@ void CorsURLLoader::SetCorsFlagIfNeeded() {
 base::Optional<std::string> CorsURLLoader::GetHeaderString(
     const ResourceResponseHead& response,
     const std::string& header_name) {
+  if (!response.headers)
+    return base::nullopt;
   std::string header_value;
   if (!response.headers->GetNormalizedHeader(header_name, &header_value))
     return base::nullopt;