Add a dummy FlagSaver class.
Add a dummy FlagSaver class.

Summary: Add a declaration to mock out FlagSaver on platforms that don't support gflags.

Reviewed By: ivmaykov, yfeldblum

Differential Revision: D3620421

fbshipit-source-id: c1047f34f384f34276b18f53e966a65cdd5c8075

facebook/folly,floxard/folly,rklabs/folly,charsyam/folly,Orvid/folly,charsyam/folly,facebook/folly,rklabs/folly,floxard/folly,floxard/folly,facebook/folly,rklabs/folly,Orvid/folly,Orvid/folly,floxard/folly,charsyam/folly,charsyam/folly,rklabs/folly,Orvid/folly,floxard/folly,Orvid/folly,rklabs/folly,facebook/folly,facebook/folly