Update expiry for ConvolverNode.ImpulseResponseLength

Extend the expiry date by 6 mo.

The response length is still useful for knowing how people use it and
for guiding possible optimizations.

Bug: 1165239
Change-Id: Iab745c5f8b3e736dc1a176eae47aa078e5061965
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625048Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843063}

tools/metrics/histograms/histograms_xml/web_audio/histograms.xml

@@ -201,7 +201,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="WebAudio.ConvolverNode.ImpulseResponseLength" units="ms"
-    expires_after="2021-02-18">
+    expires_after="2021-08-18">
   <owner>rtoy@chromium.org</owner>
   <owner>hongchan@chromium.org</owner>
   <summary>