Add arm64 to typedefs.h
Add arm64 to typedefs.h

This is the first step to get a buildable chrome_shell_apk for arm64.

BUG=chromium:354405
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/10819004

Patch from Primiano Tucci <primiano@chromium.org>.

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: fff3fd35a65363d1481321a71e98682a1d3eb9f6

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc