Allow mocking SendSideCongestionController for Call tests.

webrtc/modules/congestion_controller/BUILD.gn

Index: webrtc/modules/congestion_controller/BUILD.gn
diff --git a/webrtc/modules/congestion_controller/BUILD.gn b/webrtc/modules/congestion_controller/BUILD.gn
index 3dcbb27b24067f49966b3a732da14acfff321199..352d41083d08d0cfa2ec075759665419d91991e0 100644
--- a/webrtc/modules/congestion_controller/BUILD.gn
+++ b/webrtc/modules/congestion_controller/BUILD.gn
@@ -85,6 +85,7 @@ if (rtc_include_tests) {
     ]
     deps = [
       ":congestion_controller",
+      ":mock_congestion_controller",
       "../../base:rtc_base_approved",
       "../../system_wrappers:system_wrappers",
       "../../test:field_trial",
@@ -101,4 +102,16 @@ if (rtc_include_tests) {
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
     }
   }
+
+  rtc_source_set("mock_congestion_controller") {
+    testonly = true
+    sources = [
+      "include/mock/mock_congestion_observer.h",
+      "include/mock/mock_send_side_congestion_controller.h",
+    ]
+    deps = [
+      ":congestion_controller",
+      "../../test:test_support",
+    ]
+  }
 }