Remove legacy StartsWithASCII function.

content/child/multipart_response_delegate.cc

Index: content/child/multipart_response_delegate.cc
diff --git a/content/child/multipart_response_delegate.cc b/content/child/multipart_response_delegate.cc
index 2904d8653c86b16d78064357813043c0d7129e59..ecf7669ebebe9ffa7241707043df3161ae5e5392 100644
--- a/content/child/multipart_response_delegate.cc
+++ b/content/child/multipart_response_delegate.cc
@@ -71,7 +71,7 @@ MultipartResponseDelegate::MultipartResponseDelegate(
       stop_sending_(false),
       has_sent_first_response_(false) {
   // Some servers report a boundary prefixed with "--".  See bug 5786.
-  if (base::StartsWithASCII(boundary, "--", true)) {
+  if (base::StartsWith(boundary, "--", base::CompareCase::SENSITIVE)) {
     boundary_.assign(boundary);
   } else {
     boundary_.append(boundary);