Track transaction merges through transaction trace

Bug: 233371599
Bug: 278663063
Test: dump a transaction trace and ensure transactions have the list of merged transaction ids in the proto
Change-Id: I3edf8f1443a2573ef2086f221ceeef57172ecdbe
diff --git a/services/surfaceflinger/layerproto/transactions.proto b/services/surfaceflinger/layerproto/transactions.proto
index 2c4eb10..b0cee9b 100644
--- a/services/surfaceflinger/layerproto/transactions.proto
+++ b/services/surfaceflinger/layerproto/transactions.proto
@@ -100,6 +100,7 @@
     uint64 transaction_id = 6;
     repeated LayerState layer_changes = 7;
     repeated DisplayState display_changes = 8;
+    repeated uint64 merged_transaction_ids = 9;
 }
 
 // Keep insync with layer_state_t