ios: Fix a Wrange-loop-analysis warning.

Bug: 1039697
Change-Id: Ied0f97df085594347a8e404b91c00f8ba0933690
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2071697
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744247}

ios/chrome/browser/ui/omnibox/popup/autocomplete_match_formatter.mm

@@ -255,7 +255,7 @@ UIColor* DimColorIncognito() {
   NSMutableAttributedString* result =
       [[NSMutableAttributedString alloc] initWithString:@""];
 
-  for (const auto field : line.text_fields()) {
+  for (const auto& field : line.text_fields()) {
     [result appendAttributedString:
                 [self attributedStringForTextfield:&field
                             useDeemphasizedStyling:useDeemphasizedStyling]];