Fix flaky //third_party/http2/adapter:window_manager_test
Fix flaky //third_party/http2/adapter:window_manager_test

random_.Uniform(0) is division by zero.

10K PASS: http://sponge2/83661567-eccb-463d-9c84-e883698d97d8

PiperOrigin-RevId: 449562672

google/quiche,google/quiche,google/quiche,google/quiche