Add support creating and saving a new reference file.

third_party/WebKit/LayoutTests/webaudio/Gain/gain.html

Index: third_party/WebKit/LayoutTests/webaudio/Gain/gain.html
diff --git a/third_party/WebKit/LayoutTests/webaudio/Gain/gain.html b/third_party/WebKit/LayoutTests/webaudio/Gain/gain.html
index b948c31ff543cc6af8cd4048fa7087c22cb5e8d0..b7bf8ccc97d00886b025f0dc2a2f1b31ca1192f1 100644
--- a/third_party/WebKit/LayoutTests/webaudio/Gain/gain.html
+++ b/third_party/WebKit/LayoutTests/webaudio/Gain/gain.html
@@ -107,6 +107,9 @@ audit.define({
         });
         let snr = 10 * Math.log10(computeSNR(actual, reference));
         should(snr, 'SNR (in dB)').beGreaterThanOrEqualTo(89.088);
+        let filename = 'gain-actual.wav';
+        if (downloadAudioBuffer(buffer, filename))
+          should(true, 'Saved reference file').message(filename, "");
       })
       .then(() => task.done());
   ;