I've been running this command:
> CHROMIUM_OUTPUT_DIR=out/asan ASAN_OPTIONS="detect_leaks=1 symbolize=1 external_symbolizer_path=$SRC/third_party/llvm-build/Release+Asserts/bin/llvm-symbolizer" out/asan/content_unittests --gtest_filter="AudioFocus*StopsDucking"

Based on the command found in the doc: https://www.chromium.org/developers/testing/leaksanitizer

Which is:
> ASAN_OPTIONS="detect_leaks=1 symbolize=1 external_symbolizer_path=$SRC/third_party/llvm-build/Release+Asserts/bin/llvm-symbolizer" out/Release/base_unittests

I had no symbol and couldn't figure out why until primiano@ pointed the $SRC to me. There might be a message somewhere but with a very long output full of addresses, I've never seen it.

ON the other hand, if I try to run the same command with a symbolizer that isn't allowed like "tools/valgrind/asan/third_party/asan_symbolize.py", I get the following message:
==10045==ERROR: External symbolizer path is set to '/tools/valgrind/asan/third_party/asan_symbolize.py' which isn't a known symbolizer. Please set the path to the llvm-symbolizer binary or other known tool.

It sounds like it would be very helpful to get a similar message if the external_symbolizer_path is invalid :)