Fix flaky //third_party/http2:http2_structures_test Initialize Http2Random with a stable key. A random key causes flakes when occasionally the sequences produces two identical numbers in a row, which causes VERIFY_NE expectation to fail. 10K run: http://sponge2/eb180904-ba6f-4168-b517-bcf34d846bc4 PiperOrigin-RevId: 329772803 Change-Id: I4247596c5113aecc12a52cb5e256fa191bd1d44a