Extend expiry date for Assistant histograms

Bug: 1149455
Test: None
Change-Id: Ibf022a85132defcbbda53326da21d13287cb3210
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2546171
Commit-Queue: Yue Li <updowndota@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829897}

tools/metrics/histograms/histograms_xml/assistant/histograms.xml

@@ -62,7 +62,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Assistant.DspHotwordDetection"
-    enum="DspHotwordDetectionStatus" expires_after="2020-05-23">
+    enum="DspHotwordDetectionStatus" expires_after="2021-05-23">
   <owner>meilinw@chromium.org</owner>
   <owner>xiaohuic@chromium.org</owner>
   <summary>
@@ -90,7 +90,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Assistant.HotwordEnableNotification" enum="BooleanHit"
-    expires_after="2020-12-31">
+    expires_after="2021-05-31">
   <owner>updowndota@chromium.org</owner>
   <owner>croissant-eng@chromium.org</owner>
   <summary>
@@ -121,8 +121,8 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
-<histogram name="Assistant.OptInFlow.LoadingTimeoutCount" units="units"
-    expires_after="2020-12-31">
+<histogram name="Assistant.OptInFlow.LoadingTimeoutCount" units="timeouts"
+    expires_after="2021-05-31">
   <owner>updowndota@chromium.org</owner>
   <owner>croissant-eng@chromium.org</owner>
   <summary>