Tracking mock_paced_sender.h with a GN target

Untracked headers fly under the 'gn check' radar and in the long term
this can cause problems like unnoticed cyclic dependencies.

This cl creates a synthetic target for this header since no other
targets in webrtc/modules/pacing/BUILD.gn seem to be related to it.

BUG=webrtc:7649
NOTRY=True

Review-Url: https://codereview.webrtc.org/2887593003
Cr-Original-Commit-Position: refs/heads/master@{#19656}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 334f9e6a8d67f646b1a60a1683046224c351c5a4
4 files changed