Use descendant combinator in copyBetweenWindows browser tests

Use a decendant combinator selector to query for the USB volume: it is
shorter and more idiomatic with other volume selector JS test code.

Test: browser_test --gtest_filter="*FilesApp*copyBetweenWindows*"
Bug: 836254,851888
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I70b9ecad9e57db6a2d0028ba329707c5574710e8
Reviewed-on: https://chromium-review.googlesource.com/1098908Reviewed-by: Naoki Fukino <fukino@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567077}

ui/file_manager/integration_tests/file_manager/copy_between_windows.js

@@ -7,8 +7,7 @@
 /**
  * Query used to find USB removable volume.
  */
-var USB_VOLUME_QUERY = '#directory-tree > .tree-item > .tree-row > ' +
-    '.item-icon[volume-type-icon="removable"]';
+const USB_VOLUME_QUERY = '#directory-tree [volume-type-icon="removable"]';
 
 /**
  * Opens two window of given root paths.