FileManagerBrowserTest: directly mark CopyBetweenWindows DISABLED

This test fails on bots. Mark is DISABLED directly, rather than by the
DISABLE_SLOW_FILESAPP_TESTS macro used to control DEBUG/MSAN/ASAN (the
CopyBetweenWindows test is the only remaining test using this macro).

Add comment (conjecture about why this test is failing) and retain the
reference to issue 500966.

Tbr: fukino
No-Presubmit: true
Bug: 836254
Change-Id: I79aaac465b0fdd255a10d251cd474940c6692eb5
Reviewed-on: https://chromium-review.googlesource.com/1057548Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558288}

chrome/browser/chromeos/file_manager/file_manager_browsertest.cc

@@ -365,14 +365,9 @@ WRAPPED_INSTANTIATE_TEST_CASE_P(
                       TestParameter(IN_INCOGNITO, "openFileDialogDrive"),
                       TestParameter(NOT_IN_GUEST_MODE, "openFileDialogUnload")));
 
-#if defined(DISABLE_SLOW_FILESAPP_TESTS)
-#define MAYBE_CopyBetweenWindows DISABLED_CopyBetweenWindows
-#else
-// flaky: http://crbug.com/500966
-#define MAYBE_CopyBetweenWindows DISABLED_CopyBetweenWindows
-#endif
+// Test does too much? Flaky on all bots: http://crbug.com/500966
 WRAPPED_INSTANTIATE_TEST_CASE_P(
-    MAYBE_CopyBetweenWindows,
+    DISABLED_CopyBetweenWindows,
     FileManagerBrowserTest,
     ::testing::Values(
         TestParameter(NOT_IN_GUEST_MODE, "copyBetweenWindowsLocalToDrive"),