Move base64.h to the proper location.

Move base64.h to the proper location and put redirect header into the
old place to be able to switch downstream users on new location.

Bug: webrtc:8366
Change-Id: I5191fe631d32178d2efd1315ca9abd4250102291
Reviewed-on: https://webrtc-review.googlesource.com/88223
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24069}

rtc_base/base64_unittest.cc

diff --git a/rtc_base/base64_unittest.cc b/rtc_base/base64_unittest.cc
index f73b396..4b857f1 100644
--- a/rtc_base/base64_unittest.cc
+++ b/rtc_base/base64_unittest.cc
@@ -8,7 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#include "rtc_base/base64.h"
+#include "rtc_base/third_party/base64/base64.h"
 #include "rtc_base/gunit.h"
 #include "rtc_base/logging.h"
 #include "rtc_base/stringutils.h"