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}