[Files app] Fix module generation for u/f/f/c/j/test_importer_common.js

Follow-up to
https://chromium-review.googlesource.com/c/chromium/src/+/2579932

For the non-js module version, the `importer` variable still potentially
has to be declared.

Bug: 1133186
Change-Id: Ie5bd2e9bff24608238dc2eb2062b809f4723c719
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2584627Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836004}

ui/file_manager/file_manager/common/js/test_importer_common.js

@@ -7,10 +7,12 @@
  * @suppress {uselessCode} Temporary suppress because of the line exporting.
  */
 
-// Shared cloud importer namespace.
 // #import {importer} from './importer_common.m.js';
 // #import {TestCallRecorder} from './unittest_util.m.js';
 
+// Shared cloud importer namespace
+// eslint-disable-next-line no-var
+/* #ignore */ var importer = importer || {};
 
 /**
  * Sets up a logger for use in unit tests.  The test logger doesn't attempt to