Make mock_media_scanner available to foreground tests

Foreground import_controller_unittest needs a mock MediaScanner. Allow
background {MockMediaScanner} to be used by foreground tests.

No change in test behavior.

Tbr: slangley@
Bug: 914197
Change-Id: I146bff5e4ef5f12df5a060b3efedb1c2e1ea378e
Reviewed-on: https://chromium-review.googlesource.com/c/1373589Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615870}

ui/file_manager/file_manager/background/js/BUILD.gn

@@ -374,6 +374,8 @@ js_library("mock_media_scanner") {
     ":test_import_history",
   ]
   externs_list = [ "../../../externs/background/media_scanner.js" ]
+  visibility = []
+  visibility = [ "//ui/file_manager/file_manager/*" ]
 }
 
 js_library("media_scanner") {