Reduce dependencies on rtc::FileSystem in FileRotatingStream tests, adding helpers in webrtc::test::

webrtc/base/filerotatingstream.h

 /*
  *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 50 matching lines @@
   // Disabling buffering causes writes to block until disk is updated. This is
   // enabled by default for performance.
   bool DisableBuffering();
 
   // Returns the path used for the i-th newest file, where the 0th file is the
   // newest file. The file may or may not exist, this is just used for
   // formatting. Index must be less than GetNumFiles().
   std::string GetFilePath(size_t index) const;
 
   // Returns the number of files that will used by this stream.
-  size_t GetNumFiles() { return file_names_.size(); }
+  size_t GetNumFiles() const { return file_names_.size(); }
 
  protected:
   size_t GetMaxFileSize() const { return max_file_size_; }
 
   void SetMaxFileSize(size_t size) { max_file_size_ = size; }
 
   size_t GetRotationIndex() const { return rotation_index_; }
 
   void SetRotationIndex(size_t index) { rotation_index_ = index; }
 
@@ skipping 82 matching lines @@
 
   const size_t max_total_log_size_;
   size_t num_rotations_;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(CallSessionFileRotatingStream);
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_FILEROTATINGSTREAM_H_