[SyncFS] [Refactoring] Change RemovePrefix() to return bool

chrome/browser/sync_file_system/drive_backend/drive_backend_util.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/sync_file_system/drive_backend/drive_backend_util.h"
 
 #include "base/logging.h"
 #include "base/memory/scoped_vector.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
@@ skipping 130 matching lines @@
   if (error == -1)
     return SYNC_STATUS_NETWORK_ERROR;
 
   util::Log(logging::LOG_WARNING,
             FROM_HERE,
             "Got unexpected error: %d",
             static_cast<int>(error));
   return SYNC_STATUS_FAILED;
 }
 
-std::string RemovePrefix(const std::string& str, const std::string& prefix) {
-  if (StartsWithASCII(str, prefix, true))
-    return std::string(str.begin() + prefix.size(), str.end());
-  return str;
+bool RemovePrefix(const std::string& str, const std::string& prefix,
+                  std::string* out) {
+  if (!StartsWithASCII(str, prefix, true)) {
+    if (out)
+      *out = str;
+    return false;
+  }
+
+  if (out)
+    *out = str.substr(prefix.size());
+  return true;
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system