Change map to unordered_map for PerfParser::process_mappers_ because sorted keys are of little/no value.
Change map to unordered_map for PerfParser::process_mappers_ because sorted keys are of little/no value.

This change reduces quipper cpu time by 6%.

PiperOrigin-RevId: 174499357

google/perf_data_converter,google/perf_data_converter,google/perf_data_converter