Instrument access to WheelEvent fields.

This cl causes the bindings generator to automatically collect UKM when
these attributes are accessed. This will be used to analyze how much
entropy leaked by this API can be used for cross-site tracking.

Bug: 973801
Change-Id: Id53fa88b0181ec746d37d00677f2eeef045e300d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424847
Commit-Queue: Chris Fredrickson <cfredric@google.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809538}

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

@@ -2868,7 +2868,7 @@ enum WebFeature {
   kIdentifiabilityStudyReserved3538 = 3538,
   kIdentifiabilityStudyReserved3539 = 3539,
   kIdentifiabilityStudyReserved3540 = 3540,
-  kIdentifiabilityStudyReserved3541 = 3541,
+  kV8WheelEvent_DeltaMode_AttributeGetter = 3541,
   kIdentifiabilityStudyReserved3542 = 3542,
   kIdentifiabilityStudyReserved3543 = 3543,
   kIdentifiabilityStudyReserved3544 = 3544,

third_party/blink/renderer/core/events/wheel_event.idl

@@ -32,10 +32,10 @@
     readonly attribute double deltaX;
     readonly attribute double deltaY;
     readonly attribute double deltaZ;
-    readonly attribute unsigned long deltaMode;
+    [HighEntropy=Direct, Measure] readonly attribute unsigned long deltaMode;
 
     // Non-standard APIs
-    [MeasureAs=WheelEventWheelDeltaX] readonly attribute long wheelDeltaX;
-    [MeasureAs=WheelEventWheelDeltaY] readonly attribute long wheelDeltaY;
-    [MeasureAs=WheelEventWheelDelta] readonly attribute long wheelDelta;
+    [HighEntropy=Direct, MeasureAs=WheelEventWheelDeltaX] readonly attribute long wheelDeltaX;
+    [HighEntropy=Direct, MeasureAs=WheelEventWheelDeltaY] readonly attribute long wheelDeltaY;
+    [HighEntropy=Direct, MeasureAs=WheelEventWheelDelta] readonly attribute long wheelDelta;
 };

tools/metrics/histograms/enums.xml

@@ -29304,7 +29304,7 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3538" label="IdentifiabilityStudyReserved3538"/>
   <int value="3539" label="IdentifiabilityStudyReserved3539"/>
   <int value="3540" label="IdentifiabilityStudyReserved3540"/>
-  <int value="3541" label="IdentifiabilityStudyReserved3541"/>
+  <int value="3541" label="V8WheelEvent_DeltaMode_AttributeGetter"/>
   <int value="3542" label="IdentifiabilityStudyReserved3542"/>
   <int value="3543" label="IdentifiabilityStudyReserved3543"/>
   <int value="3544" label="IdentifiabilityStudyReserved3544"/>