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

webrtc/test/testsupport/fileutils.h

Index: webrtc/test/testsupport/fileutils.h
diff --git a/webrtc/test/testsupport/fileutils.h b/webrtc/test/testsupport/fileutils.h
index 549f96b7e72781c3fca23b3f73d891690229b0df..2135681380c0fed1cd4510c640e20a9b9cf03f63 100644
--- a/webrtc/test/testsupport/fileutils.h
+++ b/webrtc/test/testsupport/fileutils.h
@@ -71,6 +71,12 @@ std::string WorkingDir();
 // false if a file with the same name already exists.
 bool CreateDir(const std::string& directory_name);
 
+// Removes a directory, which must already be empty.
+bool RemoveDir(const std::string& directory_name);
+
+// Removes a file.
+bool RemoveFile(const std::string& file_name);
+
 // Checks if a file exists.
 bool FileExists(const std::string& file_name);