Revert "Make DriveFS opt-out in FilesApp tests."

This reverts commit e9c79d56.

Reason for revert: single_process_mash_browser_tests failing on chromium.memory/Linux ChromiumOS MSan Tests
Bug: 894799

Original change's description:
> Make DriveFS opt-out in FilesApp tests.
> 
> Bug: 891197
> Change-Id: Id3101a43cfe97e652d71f720b5f312b933af1113
> Reviewed-on: https://chromium-review.googlesource.com/c/1277133
> Reviewed-by: Noel Gordon <noel@chromium.org>
> Commit-Queue: Sam McNally <sammc@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#599117}

TBR=noel@chromium.org,sammc@chromium.org

Change-Id: If9140eb468d1e27aeb217a3202b638aa0f4e20f7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 891197
Reviewed-on: https://chromium-review.googlesource.com/c/1278757Reviewed-by: Florent Castelli <orphis@chromium.org>
Commit-Queue: Florent Castelli <orphis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599179}

chrome/browser/chromeos/file_manager/file_manager_browsertest_base.cc

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