Extend LaunchExternalAppFormSubmitHasUserGesture to 73.

The logic changes depending on this histogram have not landed,
so let us keep it around for a bit longer.

BUG=884869

Change-Id: Idad11e54709533ce11e5b154aa06b16c003ced71
Reviewed-on: https://chromium-review.googlesource.com/1256203Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595721}

tools/metrics/histograms/histograms.xml

@@ -1822,7 +1822,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Android.Intent.LaunchExternalAppFormSubmitHasUserGesture"
-    enum="Boolean" expires_after="M71">
+    enum="Boolean" expires_after="M73">
   <owner>tedchoc@chromium.org</owner>
   <owner>thildebr@chromium.org</owner>
   <summary>