message: Use unordered_set<std::string> for deprecation warnings
  This fixes the roll in Google3, otherwise it complains that
  unordered_set<const char*> hashes the pointer, which is most likely not
  what you want to do.
diff: @@ -74,7 +74,7 @@ namespace dawn_native {
  };
  struct DeviceBase::DeprecationWarnings {
  - std::unordered_set<const char*> emitted;
  + std::unordered_set<std::string> emitted;
  size_t count = 0;
  };
  