include/mock/mock_process_thread.h was not tracked by GN. This cl creates a target for it. The target is testonly because it depends on "webrtc/test:rtp_test_utils". This means that dependencies to this header cannot fly under the GN radar anymore. :) BUG=webrtc:7652 NOTRY=True Review-Url: https://codereview.webrtc.org/2881343003 Cr-Commit-Position: refs/heads/master@{#19234}