Why would we want symbol_level of 1 or less in the official release build? Since this is the variation that we ship to customers it is crucial that we be able to diagnose crashes and do profiling. Doing these effectively requires having symbols. Debugging of debug builds is certainly easier, but it is not always sufficient. And, profiling of release builds is the only type of profiling that is guaranteed to be meaningful.
Now, we shouldn't be shipping those symbols to customers, and we shouldn't let code quality be impaired by generating symbols, but I believe that all of the toolchains that we use for developing Chrome can generate fully optimized code, and symbols, and the symbols can be stripped into a separate file so that we don't ship them.
I routinely profile and debug the release versions of Chrome which means that I routinely rely on having symbols. Recommended.