Add option to force document level passive event listeners as a runtime feature.

content/public/common/content_features.cc

Index: content/public/common/content_features.cc
diff --git a/content/public/common/content_features.cc b/content/public/common/content_features.cc
index 41f8f5a4fe03dede908be36e65c13bf1acedc860..72da725670843c56b0589cd7de9f4b4ff89b5deb 100644
--- a/content/public/common/content_features.cc
+++ b/content/public/common/content_features.cc
@@ -92,6 +92,10 @@ const base::Feature kPaintOptimizations{"PaintOptimizations",
 const base::Feature kParseHTMLOnMainThread{"ParseHTMLOnMainThread",
                                            base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Whether document level event listeners should default 'passive' to true.
+const base::Feature kPassiveDocumentEventListeners{
+    "PassiveDocumentEventListeners", base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Pointer events support.
 const base::Feature kPointerEvents{"PointerEvent",
                                    base::FEATURE_DISABLED_BY_DEFAULT};