Add JoinFilename to testsupport code, replacing use of rtc::Pathname.

This is a partial revert of https://codereview.webrtc.org/2533213005,
deleting rtc::File methods accepting an rtc::Pathname argument.

Bug: webrtc:6424
Change-Id: Ib16bdc7294dbddfa12ba9ae206c024ff97e529a4
Reviewed-on: https://webrtc-review.googlesource.com/80180
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23489}
diff --git a/test/testsupport/fileutils.cc b/test/testsupport/fileutils.cc
index 23508de..7ba24c1 100644
--- a/test/testsupport/fileutils.cc
+++ b/test/testsupport/fileutils.cc
@@ -355,6 +355,11 @@
 #endif  // defined (WEBRTC_IOS)
 }
 
+std::string JoinFilename(const std::string& dir, const std::string& name) {
+  RTC_CHECK(!dir.empty()) << "Special cases not implemented.";
+  return dir + kPathDelimiter + name;
+}
+
 size_t GetFileSize(const std::string& filename) {
   FILE* f = fopen(filename.c_str(), "rb");
   size_t size = 0;