Use LowerASCII() for MIME type lowering

third_party/WebKit/Source/modules/fetch/FetchHeaderList.cpp

Index: third_party/WebKit/Source/modules/fetch/FetchHeaderList.cpp
diff --git a/third_party/WebKit/Source/modules/fetch/FetchHeaderList.cpp b/third_party/WebKit/Source/modules/fetch/FetchHeaderList.cpp
index 22da9b04d33d2a58f509ff5737729dc6ebaa0d29..7a7f88d1f59eb3a6965a0957480f639c57d6cda3 100644
--- a/third_party/WebKit/Source/modules/fetch/FetchHeaderList.cpp
+++ b/third_party/WebKit/Source/modules/fetch/FetchHeaderList.cpp
@@ -69,7 +69,7 @@ String FetchHeaderList::ExtractMIMEType() const {
     return String();
   }
   // 3. Return MIMEType, byte lowercased.
-  return mime_type.DeprecatedLower();
+  return mime_type.LowerASCII();
 }
 
 size_t FetchHeaderList::size() const {