webview: mark correct header UMA as obsolete.

Previous CL marked Android.WebView.ExtraHeaders.Valid as obsolete, which
is incorrect; the UMA which was removed was ExtraHeadersRedirect.

Bug: 1038002
Change-Id: I4508958e02a3da78edd89077efd9418f415fa7c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2553860
Auto-Submit: Richard Coles <torne@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830611}

tools/metrics/histograms/histograms_xml/android/histograms.xml

@@ -2410,9 +2410,6 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Android.WebView.ExtraHeaders.Valid" enum="BooleanValid"
     expires_after="2021-01-14">
-  <obsolete>
-    Removed Nov 2020; experiment is done and the data isn't useful.
-  </obsolete>
   <owner>torne@chromium.org</owner>
   <owner>src/android_webview/OWNERS</owner>
   <summary>
@@ -2424,6 +2421,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Android.WebView.ExtraHeadersRedirect"
     enum="WebViewExtraHeadersRedirect" expires_after="2021-02-14">
+  <obsolete>
+    Removed Nov 2020; experiment is done and the data isn't useful.
+  </obsolete>
   <owner>torne@chromium.org</owner>
   <owner>src/android_webview/OWNERS</owner>
   <summary>