This is the Alien Swarm: Reactive Drop Symbol Server.

Official builds of Alien Swarm: Reactive Drop automatically have their EXE, DLL, and PDB files added to this server.

For information on how to set up a symbol server, see Microsoft's article on Using a Symbol Server.

In this case, SymbolStoreLocation is https://symsrv.reactivedrop.com

Other public symbol servers:
https://msdl.microsoft.com/download/symbols
https://driver-symbols.nvidia.com
https://software.intel.com/sites/downloads/symbols
https://download.amd.com/dir/bin
https://chromium-browser-symsrv.commondatastorage.googleapis.com
https://symbols.mozilla.org
https://mms.alliedmods.net/symbols
https://sm.alliedmods.net/symbols