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
diff --git a/http2/http2_structures_test.cc b/http2/http2_structures_test.cc
index a837180..20dd0ec 100644
--- a/http2/http2_structures_test.cc
+++ b/http2/http2_structures_test.cc
@@ -47,7 +47,9 @@
 template <class T>
 AssertionResult VerifyRandomCalls() {
   T t1;
-  Http2Random seq1;
+  // Initialize with a stable key, to avoid test flakiness.
+  Http2Random seq1(
+      "6d9a61ddf2bc1fc0b8245505a1f28e324559d8b5c9c3268f38b42b1af3287c47");
   Randomize(&t1, &seq1);
 
   T t2;