Propagate active decode targets bitmask into DependencyDescriptor

Bug: webrtc:10342
Change-Id: I5e8a204881b94fe5786b14e27cefce2fe056e91b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178140
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31579}

modules/rtp_rtcp/source/active_decode_targets_helper.h

@@ -47,11 +47,14 @@ class ActiveDecodeTargetsHelper {
     return last_active_decode_targets_.to_ulong();
   }
 
+  std::bitset<32> ActiveChainsBitmask() const { return last_active_chains_; }
+
  private:
   // `unsent_on_chain_[i]` indicates last active decode
   // target bitmask wasn't attached to a packet on the chain with id `i`.
   std::bitset<32> unsent_on_chain_ = 0;
   std::bitset<32> last_active_decode_targets_ = 0;
+  std::bitset<32> last_active_chains_ = 0;
   int64_t last_frame_id_ = 0;
 };