[Files app] Fix open new window command

Change background page |launcher| namesspace to use var instead of const
to be accessible in the foreground page.

Test: --gtest_filter="KeyboardOperations/FilesAppBrowserTest.Test/keyboardOpenNewWindow*"
Bug: 933302
Change-Id: I6e8cec2fd96034d8376fb247743adf1818c00adf
Reviewed-on: https://chromium-review.googlesource.com/c/1477469
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633486}
  explicit TestCase(const char* name) : test_case_name(name) {
    CHECK(name) << "FATAL: no test case name";
  }

  explicit TestCase(const char* name) : test_case_name(name) {
    CHECK(name) << "FATAL: no test case name";
  }
