Log observations of explicit UPI-VPA autocomplete hints.

components/autofill/core/browser/autofill_metrics.cc

Index: components/autofill/core/browser/autofill_metrics.cc
diff --git a/components/autofill/core/browser/autofill_metrics.cc b/components/autofill/core/browser/autofill_metrics.cc
index 239334c5e660612da4f9da3d85384c3f663ff38e..05d9a8063f4bea175eee76273b48e3bbd438812d 100644
--- a/components/autofill/core/browser/autofill_metrics.cc
+++ b/components/autofill/core/browser/autofill_metrics.cc
@@ -697,6 +697,11 @@ void AutofillMetrics::LogShowedHttpNotSecureExplanation() {
 }
 
 // static
+void AutofillMetrics::LogExplicitUpiVpaHint() {
+  UMA_HISTOGRAM_BOOLEAN("Autofill.ExplicitUpiVpaHint", true);

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

We could use something like Autofill.DeveloperEngagement.UpiVpaHint so that
anyone looking up the *other* autocomplete attribute metric
"Autofill.DeveloperEngagement" will see this one as well, close by

[Roger McFarlane (Chromium), 2017/03/28 19:08:23]

Added as an Autofill.DeveloperEngagement histogram enum.

+}
+
+// static
 void AutofillMetrics::LogCardUploadDecisionUkm(
     ukm::UkmService* ukm_service,
     const GURL& url,