Extend expiration of WebVTT histogram metrics

This CL extends the expiration of the WebVTT histogram metrics, which
will be useful gauging the usage of WebVTT on the web and prioritizing
future investments in the area.

Bug: 1140006
Change-Id: I7d91609685518aed61140535766aa8cf77b16bd9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2542772Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Evan Liu <evliu@google.com>
Cr-Commit-Position: refs/heads/master@{#830726}

tools/metrics/histograms/histograms_xml/accessibility/histograms.xml

@@ -116,7 +116,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Accessibility.CaptionSettingsLoadedFromPrefs"
-    enum="BooleanEnabled" expires_after="2020-12-01">
+    enum="BooleanEnabled" expires_after="2021-12-01">
   <owner>evliu@google.com</owner>
   <owner>media-dev@chromium.org</owner>
   <summary>
@@ -126,7 +126,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Accessibility.CaptionSettingsLoadedFromSystemSettings"
-    enum="BooleanEnabled" expires_after="2020-12-01">
+    enum="BooleanEnabled" expires_after="2021-12-01">
   <owner>evliu@google.com</owner>
   <owner>media-dev@chromium.org</owner>
   <summary>
@@ -830,7 +830,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Accessibility.VTTContainsStyleBlock" enum="BooleanEnabled"
-    expires_after="2020-12-01">
+    expires_after="2021-12-01">
   <owner>evliu@google.com</owner>
   <owner>media-dev@chromium.org</owner>
   <summary>