Log observations of explicit UPI-VPA autocomplete hints.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 9f40d834e79450ea350d9ae8545cb19ae2e72de7..beb004270c1711a1a4a32545ad67752be2304ad4 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -3573,6 +3573,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Autofill.ExplicitUpiVpaHint" enum="BooleanPresent">
+  <owner>rogerm@chromium.org</owner>
+  <summary>
+    Counts the number of times the user encounters a form field that is

[Mathieu, 2017/03/23 11:26:22]

I like the "DeveloperEngagement" description:

For each form field detected, logs whether the field has an author-specified
type hint ("autocomplete") indicating UPA-VPI. 

Or some variation of that.

BTW, is that the question we want answered, or do we prefer the % of _forms_
that have a UPI-VPA hint? 

[Roger McFarlane (Chromium), 2017/03/28 19:08:24]

merged this into the developer engagement metric and updated it to count forms
having a hint, not fields having a hint.

+    explicitly tagged with autocomplete="upi-vpa".
+  </summary>
+</histogram>
+
 <histogram name="Autofill.FillDuration.FromInteraction.WithAutofill">
   <owner>isherman@chromium.org</owner>
   <summary>