Move MemoryStream to separate source files, and to a test target.

Also delete the base class MemoryStreamBase.

Bug: webrtc:6424
Change-Id: I7eec40ecff17c7c65fdf55882bccb2abaf1d5d84
Reviewed-on: https://webrtc-review.googlesource.com/c/108622
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25494}

rtc_base/sslstreamadapter_unittest.cc

@@ -17,6 +17,7 @@
 #include "rtc_base/checks.h"
 #include "rtc_base/gunit.h"
 #include "rtc_base/helpers.h"
+#include "rtc_base/memory_stream.h"
 #include "rtc_base/messagedigest.h"
 #include "rtc_base/ssladapter.h"
 #include "rtc_base/sslidentity.h"