device/fido: add some comments about the string fixup.

Walked by this function for unrelated reasons but it struck me that I
really should have added some comments when writing it.

Change-Id: Idab9e6ac0054b640b37aad1d3ec6ba88e5ffb6a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250892
Commit-Queue: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Auto-Submit: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#779612}

device/fido/get_assertion_task.cc

@@ -61,11 +61,15 @@ void GetAssertionTask::Cancel() {
 // static
 bool GetAssertionTask::StringFixupPredicate(
     const std::vector<const cbor::Value*>& path) {
+  // This filters out all elements that are not string-keyed, direct children
+  // of key 0x04, which is the `user` element of a getAssertion response.
   if (path.size() != 2 || !path[0]->is_unsigned() ||
       path[0]->GetUnsigned() != 4 || !path[1]->is_string()) {
     return false;
   }
 
+  // Of those string-keyed children, only `name` and `displayName` may have
+  // truncated UTF-8 in their values.
   const std::string& user_key = path[1]->GetString();
   return user_key == "name" || user_key == "displayName";
 }