Convert Filter::Seek() to use new callback system.

media/base/mock_filters.cc

Index: media/base/mock_filters.cc
diff --git a/media/base/mock_filters.cc b/media/base/mock_filters.cc
index da601c4e4009687307353f92fae0f1552b607585..6fe84c37255209269a80a6f3e0428479179b6976 100644
--- a/media/base/mock_filters.cc
+++ b/media/base/mock_filters.cc
@@ -169,6 +169,10 @@ void RunFilterCallback(::testing::Unused, FilterCallback* callback) {
   delete callback;
 }
 
+void RunFilterStatusCB(::testing::Unused, const FilterStatusCB& cb) {
+  cb.Run(PIPELINE_OK);
+}
+
 void RunPipelineStatusCallback(
     PipelineStatus status, PipelineStatusCallback* callback) {
   callback->Run(status);