Unless the Google contract departs from boilerplate California employment contracts in ways highly disadvantageous to one of the largest, most legally savvy firms in the state, it is unlikely that Google needs a documented reason to fire anybody, so long as they aren't retaliating against them for concrete protected concerted actions in labor organization or retaliating against them for defending their rights as a protected class under federal law.

Doubtless a real lawyer will come along here and clarify, but a good shorthand is: you can be fired in virtually any state in the union for expressing political opinions.