DL: Add a rendersubtree use counter.

This patch adds a new use counter, kRenderSubtreeAttribute, to track
usage of the feature.

R=chrishtr@chromium.org, rakina@chromium.org

Change-Id: I2d2d9e19a3853941b3fc002592f4b971db23f982
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846569Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703861}

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

@@ -2430,7 +2430,7 @@ enum WebFeature {
   kPluginElementLoadedDocument = 3046,
   kPluginElementLoadedImage = 3047,
   kPluginElementLoadedExternal = 3048,
-
+  kRenderSubtreeAttribute = 3049,
 
   // 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/dom/element.cc

@@ -2276,6 +2276,8 @@ void Element::AttributeChanged(const AttributeModificationParams& params) {
     } else if (RuntimeEnabledFeatures::DisplayLockingEnabled() &&
                name == html_names::kRendersubtreeAttr &&
                params.old_value != params.new_value) {
+      UseCounter::Count(GetDocument(), WebFeature::kRenderSubtreeAttribute);
+
       // This is needed to ensure that proper containment is put in place.
       SetNeedsStyleRecalc(kLocalStyleChange,
                           StyleChangeReasonForTracing::FromAttribute(name));

tools/metrics/histograms/enums.xml

@@ -24871,6 +24871,7 @@ Called by update_net_error_codes.py.-->
   <int value="3046" label="PluginElementLoadedDocument"/>
   <int value="3047" label="PluginElementLoadedImage"/>
   <int value="3048" label="PluginElementLoadedExternal"/>
+  <int value="3049" label="RenderSubtreeAttribute"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">