Implement the logic to find the correct <link> for Manifest in Document.

Source/core/testing/Internals.h

Index: Source/core/testing/Internals.h
diff --git a/Source/core/testing/Internals.h b/Source/core/testing/Internals.h
index 11bb77bf509fb9586828e528d688210e62ddd29c..1c3714d18680a339784f741b7ee18d82a7da9c22 100644
--- a/Source/core/testing/Internals.h
+++ b/Source/core/testing/Internals.h
@@ -53,6 +53,7 @@ class Element;
 class ExceptionState;
 class ExecutionContext;
 class GCObservation;
+class HTMLLinkElement;
 class HTMLMediaElement;
 class InternalProfilers;
 class InternalRuntimeFlags;
@@ -321,6 +322,8 @@ public:
 
     String textSurroundingNode(Node*, int x, int y, unsigned long maxLength);
 
+    HTMLLinkElement* linkManifest(Document*, ExceptionState&);
+
 private:
     explicit Internals(Document*);
     Document* contextDocument() const;