Postpone expiry of Android.NativeBackgroundTask.Task(Started|Finished)

These metrics are still needed and will be needed for the upcoming
experiment of OfflinePrefetch in Reduced Mode, which should be in M77.

Bug: 974915
Change-Id: I74ed0c3761657685dadba1f122d4a3c9c10bff8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663033
Auto-Submit: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670095}

tools/metrics/histograms/histograms.xml

@@ -2577,7 +2577,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Android.NativeBackgroundTask.TaskFinished"
-    enum="BackgroundTaskId" expires_after="2019-08-01">
+    enum="BackgroundTaskId" expires_after="2020-06-17">
   <owner>mheikal@chromium.org</owner>
   <owner>hanxi@chromium.org</owner>
   <owner>hnakashima@chromium.org</owner>
@@ -2615,7 +2615,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Android.NativeBackgroundTask.TaskStarted"
-    enum="BackgroundTaskId" expires_after="2019-08-01">
+    enum="BackgroundTaskId" expires_after="2020-06-17">
   <owner>mheikal@chromium.org</owner>
   <owner>hanxi@chromium.org</owner>
   <owner>hnakashima@chromium.org</owner>