Add UseCounter for an ad being on a page

What:
New UseCounter to tracker % of page loads that have an ad.

Why:
This metric is useful for knowing how many pageloads are applicable
for ad interventions, such as the heavy ad intervention.

Bug: 1139434
Change-Id: I8e6e1b9b6aa60afcd4d58c331c00a3b9bcf6523e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2480967
Commit-Queue: John Delaney <johnidel@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825048}

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

@@ -3053,6 +3053,7 @@ enum WebFeature {
   kXRFrameGetLightEstimate = 3724,
   kV8HTMLDialogElement_Show_Method = 3725,
   kV8HTMLDialogElement_ShowModal_Method = 3726,
+  kAdFrameDetected = 3727,
 
   // 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/frame/local_frame.cc

@@ -2050,6 +2050,8 @@ void LocalFrame::SetIsAdSubframe(blink::mojom::AdFrameType ad_frame_type) {
   ad_frame_type_ = ad_frame_type;
   UpdateAdHighlight();
   frame_scheduler_->SetIsAdFrame();
+
+  UseCounter::Count(DomWindow(), WebFeature::kAdFrameDetected);
   InstanceCounters::IncrementCounter(InstanceCounters::kAdSubframeCounter);
 }
 

tools/metrics/histograms/enums.xml

@@ -30175,6 +30175,7 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3724" label="XRFrameGetLightEstimate"/>
   <int value="3725" label="V8HTMLDialogElement_Show_Method"/>
   <int value="3726" label="V8HTMLDialogElement_ShowModal_Method"/>
+  <int value="3727" label="AdFrameDetected"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">