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}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 3f02fb7..20ba824 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -960,6 +960,8 @@
     "firewallsocketserver.h",
     "gunit.cc",
     "gunit.h",
+    "memory_stream.cc",
+    "memory_stream.h",
     "memory_usage.cc",
     "memory_usage.h",
     "natserver.cc",