Add background-wide file metadata cache.

ui/file_manager/file_manager/background/js/import_history_unittest.js

Index: ui/file_manager/file_manager/background/js/import_history_unittest.js
diff --git a/ui/file_manager/file_manager/background/js/import_history_unittest.js b/ui/file_manager/file_manager/background/js/import_history_unittest.js
index 0cde2e479b366cfc8ae059ad45701af69f5db318..8d6fdded82e216304228711b2de4c480cef7a4b9 100644
--- a/ui/file_manager/file_manager/background/js/import_history_unittest.js
+++ b/ui/file_manager/file_manager/background/js/import_history_unittest.js
@@ -321,6 +321,21 @@ function testRecordStorage_SerializingOperations(callback) {
   reportPromise(testPromise, callback);
 }
 
+function testCreateMetadataHashcode(callback) {
+  var promise =
+      importer.createMetadataHashcode(testFileEntry).then(function(hashcode) {
+        // Note that the expression matches at least 4 numbers
+        // in the last segment, since we hard code the byte
+        // size in our test file to a four digit size.
+        // In reality it will vary.
+        assertEquals(
+            0, hashcode.search(/[\-0-9]{9,}_[0-9]{4,}/),
+            'Hashcode (' + hashcode + ') does not match next pattern.');
+      });
+
+  reportPromise(promise, callback);
+}
+
 /**
  * Installs stub APIs.
  */