update .mailmap

Bug: aomedia:3223
Change-Id: Idb6c28b3651c01bfad379f6793abaee6d8d8bada
(cherry picked from commit 56a666e4b00c81dc80f325addf4d91192ee590df)

.mailmap

diff --git a/.mailmap b/.mailmap
index c2aab18..162d7fe 100644
--- a/.mailmap
+++ b/.mailmap
@@ -58,6 +58,7 @@
 Peng Bin <binpengsmail@gmail.com>
 Peng Bin <binpengsmail@gmail.com> <pengbin@kingsoft.com>
 Peter de Rivaz <peter.derivaz@gmail.com> <peter.derivaz@argondesign.com>
+Rachel Barker <rachelbarker@google.com> David Barker <david.barker@argondesign.com>
 Ralph Giles <giles@xiph.org> <giles@entropywave.com>
 Ralph Giles <giles@xiph.org> <giles@mozilla.com>
 Remya Prakasan <remya.prakasan@ittiam.com>