[Payment Request] Measure usage of languageCode.

This patch adds a use counter for PaymentAddress.languageCode to
validate that it's rarely used and can be be removed. Latest discussions
in W3C have placed it at risk in the spec:
https://github.com/w3c/payment-request/pull/764

Firefox and Safari have removed languageCode from their implementations.

Bug: 877521
Change-Id: I9e137d768be87c6dbc8affd3db3c2d6260ada46b
Reviewed-on: https://chromium-review.googlesource.com/1188489
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589289}

third_party/blink/public/platform/web_feature.mojom

@@ -1994,6 +1994,7 @@ enum WebFeature {
   kTextEncoderStreamConstructor = 2539,
   kTextDecoderStreamConstructor = 2540,
   kSignedExchangeInnerResponse = 2541,
+  kPaymentAddressLanguageCode = 2542,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/modules/payments/payment_address.idl

@@ -13,7 +13,7 @@
     readonly attribute DOMString city;
     readonly attribute DOMString country;
     readonly attribute DOMString dependentLocality;
-    readonly attribute DOMString languageCode;
+    [MeasureAs=PaymentAddressLanguageCode] readonly attribute DOMString languageCode;
     readonly attribute DOMString organization;
     readonly attribute DOMString phone;
     readonly attribute DOMString postalCode;

tools/metrics/histograms/enums.xml

@@ -19944,6 +19944,7 @@ Called by update_net_error_codes.py.-->
   <int value="2539" label="TextEncoderStreamConstructor"/>
   <int value="2540" label="TextDecoderStreamConstructor"/>
   <int value="2541" label="SignedExchangeInnerResponse"/>
+  <int value="2542" label="PaymentAddressLanguageCode"/>
 </enum>
 
 <enum name="FeaturePolicyFeature">