Update some histogram expiry dates for WebAudio

Extend the expiry date for
WebAudio.OfflineAudioContext.SampleRate384kHz and
WebAudio.ConvolverNode.ImpulseResponseLength.

Bug: 1053180, 1053181
Change-Id: Iba98dfe64cd50751cf15dc4d15fa0d8e812a6450
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062889
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742896}

tools/metrics/histograms/histograms.xml

@@ -171789,7 +171789,7 @@ regressions. -->
 </histogram>
 
 <histogram name="WebAudio.ConvolverNode.ImpulseResponseLength" units="ms"
-    expires_after="M82">
+    expires_after="2021-02-18">
   <owner>rtoy@chromium.org</owner>
   <owner>hongchan@chromium.org</owner>
   <summary>
@@ -171847,7 +171847,7 @@ regressions. -->
 </histogram>
 
 <histogram name="WebAudio.OfflineAudioContext.SampleRate384kHz" units="Hz"
-    expires_after="M82">
+    expires_after="2021-02-18">
   <owner>rtoy@chromium.org</owner>
   <owner>hongchan@chromium.org</owner>
   <summary>