These are the cheats that you can unlock by accomplishing one of the tasks from the list you see below in Time Trial mode:
Infinite grenade launcher ammunition: successfully complete Kazakhstan with a time less than 27:10. Hold Backspace and press 5.
Bulletproof Lara: successfully complete England with a time less than 27:00. Hold Backspace and press 7.
Draw enemy health: successfully complete Bolivia with a time less than 12:30. Hold Backspace and press 2.
One shot kills: successfully complete Bolivia Redux with a time less than 4:15. Hold Backspace and press 8.
Infinite assault rifle ammunition: successfully complete Japan with a time less than 12:15. Hold Backspace and press 4.
Infinite shotgun ammunition: successfully complete Ghana with a time less than 20:00. Hold Backspace and press 3.
Infinite SMG ammunition: successfully complete Peru with a time less than 21:30. Hold Backspace and press 6.
Wield Excalibur: successfully complete Nepal with a time less than 13:40. Hold Backspace and press 9.
No textures: successfully complete the game, then hold Backspace and press 1.