Add mock_video_encoder.h to api/test

This is part of the following reland cl:
https://webrtc-review.googlesource.com/c/src/+/105600

Adding just the new location of MockVideoEncoder first and updating
downstream projects before relanding the rest of that change.

Bug: webrtc:9722
Change-Id: I44ba65a72cde1eea62ee4520d8e84472f4e41c0f
Reviewed-on: https://webrtc-review.googlesource.com/c/105620
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25144}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index c420b10..0727d82 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -517,6 +517,19 @@
     ]
   }
 
+  rtc_source_set("mock_video_encoder") {
+    testonly = true
+    sources = [
+      "test/mock_video_encoder.cc",
+      "test/mock_video_encoder.h",
+    ]
+
+    deps = [
+      "../api/video_codecs:video_codecs_api",
+      "../test:test_support",
+    ]
+  }
+
   rtc_source_set("rtc_api_unittests") {
     testonly = true