What you're describing (other than that last bit) is Lamarkism which is identified by a giraffe stretching its neck out to reach the leaves of a tree. That's not how adaptation works. Mutations don't come in responses to need. Rather the mutation comes whether there is a need or not. If trees vanished from giraffe's native habitat, long necked giraffe would die out, but their short necked okapi cousins would be fine.
But if the grasslands went away and there were just forests, giraffe would be fine and okapi in a pickle.
I can't think of any scientifically fathomable reason why a lion would ever eat straw as much more than its teeth are unequipped to handle the job. Everything about its digestive system screams obligate carnivore. And everything about its physiology is adapted for predatory behavior.