It is not yet a done deal. But if Governor Gray Davis authorizes a hard-sought $30 million state-parks grant, downtown Los Angeles will soon have its first serious new green space since the City Hall lawn opened in 1928.We are talking of the Chinatown Yard Settlement agreement here, resulting from an amazing, surprise deal last week. The Trust for Public Land, with the strategic assistance of the Friends of the Los Angeles River (FOLAR), the Chinatown Yards Alliance and the Natural Resources Defense Council, got an option on 32 acres of abandoned property — mostly an old rail yard — that Mayor Riordan and Majestic Realty had recently been on the verge of re-industrializing. Crucial to the green-space takeover was huge public opposition to the development, both in the vicinity and elsewhere.