Add UseCounter metric for composited (accelerated) SVG

This patch adds a UseCounter metric for composited (accelerated) SVG
content which is enabled by the CompositeSVG experiment.

Bug: 1101002
Change-Id: I42d50caf5ae86e60c74082cb4b21846b46557390
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2498821
Auto-Submit: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820807}

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

@@ -3038,6 +3038,7 @@ enum WebFeature {
   kColorSchemeDarkSupportedOnRoot = 3707,
   kWebBluetoothGetAvailability = 3708,
   kDigitalGoodsListPurchases = 3709,
+  kCompositedSVG = 3710,
 
   // 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/layout/svg/layout_svg_root.cc

@@ -625,6 +625,9 @@ bool LayoutSVGRoot::HasDescendantWithCompositingReason() const {
       }
     }
     has_descendant_with_compositing_reason_dirty_ = false;
+
+    if (has_descendant_with_compositing_reason_)
+      UseCounter::Count(GetDocument(), WebFeature::kCompositedSVG);
   }
   return has_descendant_with_compositing_reason_;
 }

third_party/blink/renderer/core/layout/svg/layout_svg_root_test.cc

@@ -219,4 +219,25 @@ TEST_P(CompositeSVGLayoutSVGRootTest, HasDescendantCompositingReasons) {
   EXPECT_FALSE(root.HasDescendantCompositingReasons());
 }
 
+TEST_P(CompositeSVGLayoutSVGRootTest, CompositedSVGMetric) {
+  SetBodyInnerHTML(R"HTML(
+    <style>
+      .anim { animation: anim 5s infinite; }
+      @keyframes anim {
+        from { transform: translateX(0); }
+        to { transform: translateX(100px); }
+      }
+    </style>
+    <svg style="width: 200px; height: 200px;">
+      <rect id="rect" width="100" height="100" fill="green"/>
+    </svg>
+  )HTML");
+
+  EXPECT_FALSE(GetDocument().IsUseCounted(WebFeature::kCompositedSVG));
+  auto* rect = GetDocument().getElementById("rect");
+  rect->setAttribute(html_names::kClassAttr, "anim");
+  UpdateAllLifecyclePhasesForTest();
+  EXPECT_TRUE(GetDocument().IsUseCounted(WebFeature::kCompositedSVG));
+}
+
 }  // namespace blink

tools/metrics/histograms/enums.xml

@@ -30006,6 +30006,7 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3707" label="ColorSchemeDarkSupportedOnRoot"/>
   <int value="3708" label="WebBluetoothGetAvailability"/>
   <int value="3709" label="DigitalGoodsListPurchases"/>
+  <int value="3710" label="CompositedSVG"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">