Add ebuild variable for TTS cache path if existed.

BUG=b:229014327
TEST=./run_tests && manual testing with cros_mark_android_as_stable and
emerge-hatch chromeos-base/android-vm-rvc.

Change-Id: I26d7877557a18ac1e842127f9e45cc7a049bf83b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3621638
Reviewed-by: Yury Khmel <khmel@google.com>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Hung Nguyen <hungmn@google.com>
Commit-Queue: Hung Nguyen <hungmn@google.com>
diff --git a/scripts/cros_mark_android_as_stable.py b/scripts/cros_mark_android_as_stable.py
index 757cae7..c23bf75 100644
--- a/scripts/cros_mark_android_as_stable.py
+++ b/scripts/cros_mark_android_as_stable.py
@@ -150,7 +150,7 @@
   """
   variables = {}
 
-  buckets = ['ureadahead_pack', 'gms_core_cache']
+  buckets = ['ureadahead_pack', 'gms_core_cache', 'tts_cache']
   archs = ['arm', 'arm64', 'x86', 'x86_64']
   build_types = ['user', 'userdebug']