Measure usage of only CredentialRequestOptions.unmediated

This change adds a counter to measure usages of only providing
CredentialRequestOptions.unmediated.

BUG=721399

Review-Url: https://codereview.chromium.org/2899763002
Cr-Commit-Position: refs/heads/master@{#473858}

third_party/WebKit/Source/core/frame/UseCounter.h

@@ -1602,6 +1602,7 @@ class CORE_EXPORT UseCounter {
     kShapeDetection_BarcodeDetectorConstructor = 1991,
     kShapeDetection_FaceDetectorConstructor = 1992,
     kShapeDetection_TextDetectorConstructor = 1993,
+    kCredentialManagerCredentialRequestOptionsOnlyUnmediated = 1994,
 
     // Add new features immediately above this line. Don't change assigned
     // numbers of any item, and don't reuse removed slots.

third_party/WebKit/Source/modules/credentialmanager/CredentialsContainer.cpp

@@ -181,6 +181,9 @@ ScriptPromise CredentialsContainer::get(
   String mediation = "optional";
   if (options.hasUnmediated() && !options.hasMediation()) {
     mediation = options.unmediated() ? "silent" : "optional";
+    UseCounter::Count(
+        context,
+        UseCounter::kCredentialManagerCredentialRequestOptionsOnlyUnmediated);
   } else if (options.hasMediation()) {
     mediation = options.mediation();
     if (options.hasUnmediated() &&

tools/metrics/histograms/enums.xml

@@ -15008,6 +15008,8 @@ uploading your change for review.  These are checked by presubmit scripts.
   <int value="1991" label="ShapeDetection_BarcodeDetectorConstructor"/>
   <int value="1992" label="ShapeDetection_FaceDetectorConstructor"/>
   <int value="1993" label="ShapeDetection_TextDetectorConstructor"/>
+  <int value="1994"
+      label="CredentialManagerCredentialRequestOptionsOnlyUnmediated"/>
 </enum>
 
 <enum name="FeedbackSource" type="int">