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

quiche/http2/adapter/window_manager_test.cc

diff --git a/quiche/http2/adapter/window_manager_test.cc b/quiche/http2/adapter/window_manager_test.cc
index 0e05b85..f6617c5 100644
--- a/quiche/http2/adapter/window_manager_test.cc
+++ b/quiche/http2/adapter/window_manager_test.cc
@@ -72,7 +72,9 @@
     wm_.MarkDataBuffered(buffered);
     total_buffered += buffered;
     EXPECT_TRUE(call_sequence_.empty());
-    int64_t flushed = random_.Uniform(total_buffered - total_flushed);
+    int64_t flushed = (total_buffered - total_flushed) > 0
+                          ? random_.Uniform(total_buffered - total_flushed)
+                          : 0;
     wm_.MarkDataFlushed(flushed);
     total_flushed += flushed;
   }