Part 5.1: Is policy list subsumed under subsuming policy?

third_party/WebKit/Source/core/frame/csp/MediaListDirective.cpp

Index: third_party/WebKit/Source/core/frame/csp/MediaListDirective.cpp
diff --git a/third_party/WebKit/Source/core/frame/csp/MediaListDirective.cpp b/third_party/WebKit/Source/core/frame/csp/MediaListDirective.cpp
index c067b3ce3c9ee6c6dcda9668f741be9e57aa87db..b031f79ec1219ee8779a0a5c98d1060f0645460a 100644
--- a/third_party/WebKit/Source/core/frame/csp/MediaListDirective.cpp
+++ b/third_party/WebKit/Source/core/frame/csp/MediaListDirective.cpp
@@ -81,4 +81,37 @@ void MediaListDirective::parse(const UChar* begin, const UChar* end) {
   }
 }
 
+bool MediaListDirective::subsumes(
+    const std::vector<MediaListDirective*>& other) {
+  if (!other.size())
+    return false;
+
+  // Find the effective set of plugins allowed by `other`.
+  HashSet<String> normalizedB = other[0]->m_pluginTypes;
+  for (size_t i = 1; i < other.size(); i++)
+    normalizedB = other[i]->getIntersect(normalizedB);
+
+  // Empty list of plugins is equivalent to no plugins being allowed.
+  if (!m_pluginTypes.size())
+    return !normalizedB.size();
+
+  // Check that each element of `normalizedB` is allowed by `m_pluginTypes`.
+  for (auto it = normalizedB.begin(); it != normalizedB.end(); ++it) {
+    if (!allows(*it))
+      return false;
+  }
+
+  return true;
+}
+
+HashSet<String> MediaListDirective::getIntersect(const HashSet<String>& other) {
+  HashSet<String> normalized;
+  for (const auto& type : m_pluginTypes) {
+    if (other.contains(type))
+      normalized.add(type);
+  }
+
+  return normalized;
+}
+
 }  // namespace blink