Bump expires_after date of Previews.ServerLitePage.URLLoader.Attempted

This metric is for a feature that hasn't fully launched yet and is still
actively being tracked.

Bug: 969949
Change-Id: I526aef04178f3db728f3e087fb81e28180095deb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1641462
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665993}

tools/metrics/histograms/histograms.xml

@@ -100175,7 +100175,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Previews.ServerLitePage.URLLoader.Attempted"
-    enum="BooleanAttempted" expires_after="2019-08-30">
+    enum="BooleanAttempted" expires_after="2020-08-30">
   <owner>ryansturm@chromium.org</owner>
   <summary>
     Whether or not the server lite page preview was attempted during a