Add UseCounter for ImperativeSlotAPI

This previously had no use counter.

I'm adding NOTRY here because branch day seems to be an
impossible day to land changes to enums.xml. Hopefully this
sails through. Feel free to revert if this breaks something.
NOTRY=true

Bug: 869308
Change-Id: Ic81d9620f34b8fe0998f8f6e7cb9d209b028e6a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367474
Commit-Queue: Yu Han <yuzhehan@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800328}

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

@@ -2768,6 +2768,7 @@ enum WebFeature {
   kRTCIceCandidatePort = 3439,
   kRTCIceCandidateRelatedAddress = 3440,
   kRTCIceCandidateRelatedPort = 3441,
+  kSlotAssignNode = 3442,
 
   // 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/core/html/html_slot_element.cc

@@ -196,6 +196,7 @@ void HTMLSlotElement::assign(HeapVector<Member<Node>> nodes,
   if (!CheckNodesValidity(nodes, exception_state))
     return;
 
+  UseCounter::Count(GetDocument(), WebFeature::kSlotAssignNode);
   ContainingShadowRoot()->GetSlotAssignment().ClearCandidateNodes(
       assigned_nodes_candidates_);
   HeapLinkedHashSet<Member<Node>> candidates;

tools/metrics/histograms/enums.xml

@@ -28873,6 +28873,7 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3439" label="RTCIceCandidatePort"/>
   <int value="3440" label="RTCIceCandidateRelatedAddress"/>
   <int value="3441" label="RTCIceCandidateRelatedPort"/>
+  <int value="3442" label="SlotAssignNode"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">