Extend Blink.Paint.Cahched* to 2021-7-1

They are useful when we finch CompositeAfterPaint.

Bug: 1167827
Change-Id: I51fa75f3c32ecbdbff28f199cca15974bedcb4d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2643273Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846212}

tools/metrics/histograms/histograms_xml/blink/histograms.xml

@@ -1804,7 +1804,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Blink.Paint.CachedItemPercentage" units="%"
-    expires_after="2021-03-01">
+    expires_after="2021-07-01">
   <owner>wangxianzhu@chromium.org</owner>
   <owner>paint-dev@chromium.org</owner>
   <summary>
@@ -1815,7 +1815,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Blink.Paint.CachedSubsequencePercentage" units="%"
-    expires_after="2021-03-01">
+    expires_after="2021-07-01">
   <owner>wangxianzhu@chromium.org</owner>
   <owner>paint-dev@chromium.org</owner>
   <summary>