Ignore unknown-pragmas warning of clang This was previously ignored through Chromium until https://chromium-review.googlesource.com/984113 Bug: None Change-Id: I4bc402e32d888fc2536ad69c832130c4086b5adb Reviewed-on: https://webrtc-review.googlesource.com/66040 Reviewed-by: Patrik Höglund <phoglund@webrtc.org>