Add scan mode to choose whether to do content scan or only history check.

ui/file_manager/file_manager/common/js/importer_common.js

Index: ui/file_manager/file_manager/common/js/importer_common.js
diff --git a/ui/file_manager/file_manager/common/js/importer_common.js b/ui/file_manager/file_manager/common/js/importer_common.js
index 550cc975228fba0a6917ccae52a7e5bfa8a1bf81..700486d7b79da76deb090c4221eb0f3daa704bc0 100644
--- a/ui/file_manager/file_manager/common/js/importer_common.js
+++ b/ui/file_manager/file_manager/common/js/importer_common.js
@@ -13,6 +13,19 @@ importer.ScanEvent = {
 };
 
 /**
+ * Mode of the scan to find new files.
+ *
+ * @enum {string}
+ */
+importer.ScanMode = {
+  // Faster scan using import history to get candidates of new files.
+  HISTORY: 'HISTORY',
+  // Verifies content hash to eliminate content duplications from candidates
+  // chosen by HISTORY.
+  CONTENT: 'CONTENT',
+};
+
+/**
  * Disposition of an entry with respect to it's
  * presence in import history, drive, and so on.
  * @enum {string}