Background tracing: fixed merging of metadata dict used for HTTP POST headers

R=ssid@chromium.org
BUG=1017462

Change-Id: I22e281df59c29436ac092b9d4c959d9856a6b201
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876978
Auto-Submit: oysteine <oysteine@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708830}

services/tracing/public/cpp/perfetto/trace_event_data_source.cc

@@ -246,7 +246,7 @@ TraceEventMetadataSource::GenerateLegacyMetadataDict() {
     if (!metadata_dict) {
       continue;
     }
-    merged_metadata->MergeDictionary(merged_metadata.get());
+    merged_metadata->MergeDictionary(metadata_dict.get());
   }
 
   base::trace_event::MetadataFilterPredicate metadata_filter =