Temporary remove UNREACHED from ProtoWriter::AppendAsTraceFormat()

This method was not expected to be called because TracedValues are
stored and written in proto format only. But the export to atrace
format still occasionally calls it. Since we're migrating to the
Perfetto SDK now and the atrace export will be reimplemented for this
purpose, it makes sense to just disable the warning for now.

Bug: 1411929, 1006541
Change-Id: I9b1d99f8adc18b4bb0181cfa4261c364170f8275
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4221236
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Mikhail Khokhlov <khokhlov@google.com>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1101511}
diff --git a/services/tracing/public/cpp/perfetto/traced_value_proto_writer.cc b/services/tracing/public/cpp/perfetto/traced_value_proto_writer.cc
index 9efd4d7..601ad7e 100644
--- a/services/tracing/public/cpp/perfetto/traced_value_proto_writer.cc
+++ b/services/tracing/public/cpp/perfetto/traced_value_proto_writer.cc
@@ -197,7 +197,12 @@
     return full_size;
   }
 
-  void AppendAsTraceFormat(std::string* out) const override { NOTREACHED(); }
+  void AppendAsTraceFormat(std::string* /*out*/) const override {
+    // This is not implemented, but still occasionally called from atrace
+    // exporter. See crbug.com/1411929 for details.
+    // TODO(khokhlov): Make sure this method in NOTREACHED() after the SDK
+    // migration (crbug.com/1006541).
+  }
 
   bool AppendToProto(
       base::trace_event::TracedValue::ProtoAppender* appender) override {