Switch base::Value typemapping to be by value instead of by unique_ptr.

extensions/browser/sandboxed_unpacker.h

Index: extensions/browser/sandboxed_unpacker.h
diff --git a/extensions/browser/sandboxed_unpacker.h b/extensions/browser/sandboxed_unpacker.h
index 5b081df194529f223418b11ef554625d78d37477..165a380a580f6b02073084c4e883fe9d40211ac4 100644
--- a/extensions/browser/sandboxed_unpacker.h
+++ b/extensions/browser/sandboxed_unpacker.h
@@ -206,7 +206,7 @@ class SandboxedUnpacker : public base::RefCountedThreadSafe<SandboxedUnpacker> {
   // Unpacks the extension in directory and returns the manifest.
   void Unpack(const base::FilePath& directory);
   void UnpackDone(const base::string16& error,
-                  std::unique_ptr<base::DictionaryValue> manifest);
+                  const base::Optional<base::DictionaryValue>& manifest);
   void UnpackExtensionSucceeded(
       std::unique_ptr<base::DictionaryValue> manifest);
   void UnpackExtensionFailed(const base::string16& error);