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..5f799e58df209332f0fb4ed23a8c86c1da791f3a 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 +108,13 @@ ResourceRequest createAccessControlPreflightRequest(
         HTTPNames::Access_Control_Request_External, "true");
   }
 
-  if (request.httpHeaderFields().size() > 0) {
-    preflightRequest.setHTTPHeaderField(
-        HTTPNames::Access_Control_Request_Headers,
-        createAccessControlRequestHeadersHeader(request.httpHeaderFields()));
+  if (request.httpHeaderFields().size()) {

[yhirano, 2017/01/18 05:41:55]

Do we need this outer branch?

[sof, 2017/01/18 06:57:48]

It almost always holds, i reckon & the extra code it executes isn't worth losing
sleep over, so removed.

+    AtomicString headers =
+        createAccessControlRequestHeadersHeader(request.httpHeaderFields());
+    if (headers != nullAtom) {
+      preflightRequest.setHTTPHeaderField(
+          HTTPNames::Access_Control_Request_Headers, headers);
+    }
   }
 
   return preflightRequest;