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

webrtc/base/filerotatingstream.h

Index: webrtc/base/filerotatingstream.h
diff --git a/webrtc/base/filerotatingstream.h b/webrtc/base/filerotatingstream.h
index a8522ff0c5f01d672d7e265a9a797cab0d59d677..a3e808cda52670f7e9e3b67294bab9b9b6838318 100644
--- a/webrtc/base/filerotatingstream.h
+++ b/webrtc/base/filerotatingstream.h
@@ -68,7 +68,7 @@ class FileRotatingStream : public StreamInterface {
   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_; }