libbrillo: Deprecated comment for brillo::Minijail

Add comment stating |brillo::Minijail| is deprecated.
Add TODO to completely deprecate |brillo::Minijail| + its use cases.

BUG=chromium:992135
TEST=FEATURES=test emerge-$B libbrillo

Change-Id: Id3034a5b3009e5bf6f3fbf95475cd22ba48bc6df
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2485736
Reviewed-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/libbrillo/brillo/minijail/minijail.h b/libbrillo/brillo/minijail/minijail.h
index e0a0d72..3b8d756 100644
--- a/libbrillo/brillo/minijail/minijail.h
+++ b/libbrillo/brillo/minijail/minijail.h
@@ -19,6 +19,10 @@
 
 namespace brillo {
 
+// DEPRECATED. Use libminijail.h instead.
+// TODO(crbug.com/992135): Deprecate |Minijail| with attribute and completely
+// remove all use cases.
+//
 // A Minijail abstraction allowing Minijail mocking in tests.
 class BRILLO_EXPORT Minijail {
  public: