Leave out empty-valued Access-Control-Request-Headers: on preflights.

third_party/WebKit/Source/core/fetch/CrossOriginAccessControl.cpp

Index: third_party/WebKit/Source/core/fetch/CrossOriginAccessControl.cpp
diff --git a/third_party/WebKit/Source/core/fetch/CrossOriginAccessControl.cpp b/third_party/WebKit/Source/core/fetch/CrossOriginAccessControl.cpp
index 2e8c98ac2ed9b288e0eb1e83285c9cf87c8de604..ff43d8812f29a747f37b100e4e7763b396c61127 100644
--- a/third_party/WebKit/Source/core/fetch/CrossOriginAccessControl.cpp
+++ b/third_party/WebKit/Source/core/fetch/CrossOriginAccessControl.cpp
@@ -70,6 +70,8 @@ static AtomicString createAccessControlRequestHeadersHeader(
     }
     filteredHeaders.push_back(header.key.lower());
   }
+  if (!filteredHeaders.size())
+    return nullAtom;
 
   // Sort header names lexicographically.
   std::sort(filteredHeaders.begin(), filteredHeaders.end(),
@@ -106,10 +108,11 @@ ResourceRequest createAccessControlPreflightRequest(
         HTTPNames::Access_Control_Request_External, "true");
   }
 
-  if (request.httpHeaderFields().size() > 0) {
+  AtomicString requestHeaders =
+      createAccessControlRequestHeadersHeader(request.httpHeaderFields());
+  if (requestHeaders != nullAtom) {
     preflightRequest.setHTTPHeaderField(
-        HTTPNames::Access_Control_Request_Headers,
-        createAccessControlRequestHeadersHeader(request.httpHeaderFields()));
+        HTTPNames::Access_Control_Request_Headers, requestHeaders);
   }
 
   return preflightRequest;