crypto/tls: reduce size of buffer in throughput benchmarks

The Windows builders run the throughput benchmarks really slowly with a
64kb buffer. Lowering it to 16kb brings the performance back into line
with the other builders.

This is a work-around to get the build green until we can figure out why
the Windows builders are slow with the larger buffer size.

Update #15899

Change-Id: I215ebf115e8295295c87f3b3e22a4ef1f9e77f81
Reviewed-on: https://go-review.googlesource.com/23574Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/crypto/tls/tls_test.go

@@ -504,7 +504,7 @@ func throughput(b *testing.B, totalBytes int64, dynamicRecordSizingDisabled bool
 	clientConfig := *testConfig
 	clientConfig.DynamicRecordSizingDisabled = dynamicRecordSizingDisabled
 
-	buf := make([]byte, 1<<16)
+	buf := make([]byte, 1<<14)
 	chunks := int(math.Ceil(float64(totalBytes) / float64(len(buf))))
 	for i := 0; i < N; i++ {
 		conn, err := Dial("tcp", ln.Addr().String(), &clientConfig)