[Push] Change of deprecated function call

In base/values.h, Value::GetStringWithoutPathExpansion is deprecated.
Call Value::FindStringKey(key) instead.

Change-Id: I037894b4dc52602e55cd60351c4163c4009ad9f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2310436Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Commit-Queue: Viviane Yang <viviy@google.com>
Cr-Commit-Position: refs/heads/master@{#790787}

chrome/browser/push_messaging/push_messaging_app_identifier.cc

@@ -129,13 +129,14 @@ PushMessagingAppIdentifier PushMessagingAppIdentifier::FindByAppId(
   const base::DictionaryValue* map =
       profile->GetPrefs()->GetDictionary(prefs::kPushMessagingAppIdentifierMap);
 
-  std::string map_value;
-  if (!map->GetStringWithoutPathExpansion(app_id, &map_value))
+  const std::string* map_value = map->FindStringKey(app_id);
+
+  if (!map_value || map_value->empty())
     return PushMessagingAppIdentifier();
 
   GURL origin;
   int64_t service_worker_registration_id;
-  if (!GetOriginAndSWRFromPrefValue(map_value, &origin,
+  if (!GetOriginAndSWRFromPrefValue(*map_value, &origin,
                                     &service_worker_registration_id)) {
     NOTREACHED();
     return PushMessagingAppIdentifier();