Update to latest google3 version

[jmr]

• Use absl::flags and absl::log instead of gflags/glog
• Use ABSL_UNREACHABLE, requires https://github.com/abseil/abseil-cpp/commit/6a87605
• New edge/circle intersection ordering predicates
• New functions GetUVCoordOfEdge and GetIJCoordOfEdge
• S2PolylineAlignment: Use Norm instead of Norm2
• S2Polygon::DestructiveUnion: Use priority_queue internally
• Update CONTRIBUTING.md with more detailed instructions
• Replace const char * with absl::string_view
• Replace most gtl::dense_hash_set uses with absl::flat_hash_set
• Replace most std::set and std::map with absl::flat_hash_set and flat_hash_map

[jmr]

@MBkkt