[Mac] Produce correct DomKey when Ctrl/Shift/Command is down

ui/events/keycodes/keyboard_code_conversion_mac.mm

Index: ui/events/keycodes/keyboard_code_conversion_mac.mm
diff --git a/ui/events/keycodes/keyboard_code_conversion_mac.mm b/ui/events/keycodes/keyboard_code_conversion_mac.mm
index f435f4ed2b966d19a8524ccc2fe63ef61c93c91f..bfc7e725a99412da8ed0d75c52389ee446bf8df5 100644
--- a/ui/events/keycodes/keyboard_code_conversion_mac.mm
+++ b/ui/events/keycodes/keyboard_code_conversion_mac.mm
@@ -746,8 +746,22 @@ DomKey DomKeyFromNSEvent(NSEvent* event) {
   // doesn't.
   if ([event type] == NSKeyDown || [event type] == NSKeyUp) {
     NSString* characters = [event characters];
-    if ([characters length] > 0)
-      return DomKeyFromCharCode([characters characterAtIndex:0]);
+    if ([characters length] > 0) {
+      unichar dom_key_char =
+          [characters characterAtIndex:[characters length] - 1];
+      if ([event modifierFlags] != 0 && std::iscntrl(dom_key_char)) {
+        // According to spec if the key combination produces a non-printable
+        // character, the key value should be the character without modifiers.
+        // See https://w3c.github.io/uievents/#keys-guidelines
+        NSString* unmodified_characters = [event charactersIgnoringModifiers];
+        if ([unmodified_characters length] > 0) {
+          unichar unmodified_char = [unmodified_characters
+              characterAtIndex:[unmodified_characters length] - 1];
+          dom_key_char = unmodified_char;
+        }
+      }
+      return DomKeyFromCharCode(dom_key_char);
+    }
   }
   return DomKeyFromKeyCode([event keyCode]);
 }