[webnfc] device.mojom.NDEFRecord.record_type could never be null

BUG=520391

Change-Id: I09a15805e86d2c1a933295587af9a2a99e6de5df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900806
Auto-Submit: Leon Han <leon.han@intel.com>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712933}

services/device/public/mojom/nfc.mojom

@@ -37,7 +37,7 @@ struct NDEFError {
 // https://w3c.github.io/web-nfc/#dom-ndefrecord
 struct NDEFRecord {
   // The type of NDEFRecord.
-  string? record_type;
+  string record_type;
 
   // Represents the IANA media type of the NDEFRecord data field.
   string? media_type;