FileUtilsTest DirExists function added

BUG=webrtc:7218

Review-Url: https://codereview.webrtc.org/2749163003
Cr-Commit-Position: refs/heads/master@{#17247}
diff --git a/webrtc/test/testsupport/fileutils.h b/webrtc/test/testsupport/fileutils.h
index 1a5f9bb..549f96b 100644
--- a/webrtc/test/testsupport/fileutils.h
+++ b/webrtc/test/testsupport/fileutils.h
@@ -74,6 +74,9 @@
 // Checks if a file exists.
 bool FileExists(const std::string& file_name);
 
+// Checks if a directory exists.
+bool DirExists(const std::string& directory_name);
+
 // File size of the supplied file in bytes. Will return 0 if the file is
 // empty or if the file does not exist/is readable.
 size_t GetFileSize(const std::string& filename);