This header was not tracked by a GN target and in the discussion on
https://bugs.chromium.org/p/webrtc/issues/detail?id=7617 we decided
to also move it under webrtc/base.
I checked in chromium code search and it seems safe to move it
without creating a stub header in webrtc/system_wrappers.
BUG=webrtc:7617
Review-Url: https://codereview.webrtc.org/2882673002
Cr-Commit-Position: refs/heads/master@{#18151}