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-Commit-Position: refs/heads/master@{#19656}

webrtc/call/BUILD.gn

@@ -182,6 +182,7 @@ if (rtc_include_tests) {
       "../modules/bitrate_controller",
       "../modules/congestion_controller:mock_congestion_controller",
       "../modules/pacing",
+      "../modules/pacing:mock_paced_sender",
       "../modules/rtp_rtcp",
       "../modules/rtp_rtcp:mock_rtp_rtcp",
       "../modules/utility:mock_process_thread",

webrtc/modules/bitrate_controller/BUILD.gn

@@ -61,6 +61,7 @@ if (rtc_include_tests) {
       ":bitrate_controller",
       "../../test:field_trial",
       "../../test:test_support",
+      "../pacing:mock_paced_sender",
       "../remote_bitrate_estimator:remote_bitrate_estimator",
       "//testing/gmock",
     ]

webrtc/modules/congestion_controller/BUILD.gn

@@ -97,6 +97,7 @@ if (rtc_include_tests) {
       "../../test:field_trial",
       "../../test:test_support",
       "../bitrate_controller:bitrate_controller",
+      "../pacing:mock_paced_sender",
       "../pacing:pacing",
       "../remote_bitrate_estimator:remote_bitrate_estimator",
       "../rtp_rtcp:rtp_rtcp",

webrtc/modules/pacing/BUILD.gn

@@ -75,4 +75,16 @@ if (rtc_include_tests) {
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
     }
   }
+
+  rtc_source_set("mock_paced_sender") {
+    testonly = true
+    sources = [
+      "mock/mock_paced_sender.h",
+    ]
+    deps = [
+      ":pacing",
+      "../../system_wrappers",
+      "../../test:test_support",
+    ]
+  }
 }