Make DriveFS opt-out in FilesApp tests.

Bug: 891197
Change-Id: Id3101a43cfe97e652d71f720b5f312b933af1113
Reviewed-on: https://chromium-review.googlesource.com/c/1277133Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599117}

chrome/browser/chromeos/file_manager/file_manager_browsertest_base.cc

@@ -1141,7 +1141,7 @@ bool FileManagerBrowserTestBase::GetTabletMode() const {
 }
 
 bool FileManagerBrowserTestBase::GetEnableDriveFs() const {
-  return false;
+  return true;
 }
 
 bool FileManagerBrowserTestBase::GetRequiresStartupBrowser() const {