profiling: Use std::string::rfind for OS X memrchr.
profiling: Use std::string::rfind for OS X memrchr.

Change-Id: I41dbdc17a91ab63787323fbb59909c6f43dbec90
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto