Instrument AuthenticatorAttestationResponse.getTransports() for identifiability study.

Bug: 973801
Change-Id: I1d19acc974abf0fdad068296339d719c51b6b129
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368466
Commit-Queue: Dylan Cutler <dylancutler@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801488}

third_party/blink/public/mojom/web_feature/web_feature.mojom

@@ -2780,6 +2780,7 @@ enum WebFeature {
   kRTCPeerConnectionSetLocalDescription = 3452,
   kRTCPeerConnectionCreateOffer = 3453,
   kRTCPeerConnectionCreateAnswer = 3454,
+  kV8AuthenticatorAttestationResponse_GetTransports_Method = 3455,
 
   // 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/credentialmanager/authenticator_attestation_response.idl

@@ -10,7 +10,7 @@
     Exposed=Window
 ] interface AuthenticatorAttestationResponse : AuthenticatorResponse {
     [SameObject] readonly attribute ArrayBuffer attestationObject;
-    sequence<DOMString> getTransports();
+    [HighEntropy=Direct, Measure] sequence<DOMString> getTransports();
 
     ArrayBuffer getAuthenticatorData();
     ArrayBuffer? getPublicKey();

tools/metrics/histograms/enums.xml

@@ -28927,6 +28927,8 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3452" label="RTCPeerConnectionSetLocalDescription"/>
   <int value="3453" label="RTCPeerConnectionCreateOffer"/>
   <int value="3454" label="RTCPeerConnectionCreateAnswer"/>
+  <int value="3455"
+      label="V8AuthenticatorAttestationResponse_GetTransports_Method"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">