[filesapp] Re-enable directoryTreeExpandHorizontalScrollRTL test

This reverts commit 421d2ad9.
The test was failing on linux-chromeos-chrome internal bot because
the CSSOMViewScrollCoordinates flag had not been enabled yet on it.
However, that should no longer be an issue after [1].

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2310799

Original change's description:
> Disable DirectoryTree/FilesAppBrowserTest.Test/directoryTreeExpandHorizontalScrollRTL
>
> Failin on linux-chromeos-chrome internal bot since
> https://ci.chromium.org/p/chrome/builders/ci/linux-chromeos-chrome/3955
>
> Error like "[FAIL] [directoryTreeExpandHorizontalScrollRTL]: Tree should not scroll right
>
> This is blocking src-internal rolls (for >1 day).
>
> Tbr: majidvp@chromium.org, noel@chromium.org
> Bug: 1061821, 1061670
> Change-Id: I27dd3fca5a5d9154519680b748c6dd054f2dd021
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2103012
> Reviewed-by: Trent Apted <tapted@chromium.org>
> Reviewed-by: Noel Gordon <noel@chromium.org>
> Commit-Queue: Trent Apted <tapted@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#750465}

Bug: 1061821, 1061670
Change-Id: I12f490679d9d465b676318722282ef5f9fb190dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377489Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802056}

chrome/browser/chromeos/file_manager/file_manager_browsertest.cc

@@ -526,8 +526,7 @@ WRAPPED_INSTANTIATE_TEST_SUITE_P(
         TestCase("directoryTreeRecentsSubtypeScroll").EnableUnifiedMediaView(),
         TestCase("directoryTreeHorizontalScroll"),
         TestCase("directoryTreeExpandHorizontalScroll"),
-        // Disabled. Fails on internal ChromeOS bot. https://crbug.com/1061821.
-        // TestCase("directoryTreeExpandHorizontalScrollRTL"),
+        TestCase("directoryTreeExpandHorizontalScrollRTL"),
         TestCase("directoryTreeVerticalScroll"),
         TestCase("directoryTreeExpandFolder")));