tast: add new test dependency `selinux'.

BUG=b:114172200
TEST=none
CQ-DEPEND=CL:1208774

Change-Id: I7b1bfa4ff557edd2e20401e39f2c25629e60aca7
Reviewed-on: https://chromium-review.googlesource.com/1212644
Commit-Ready: Qijiang Fan <[email protected]>
Tested-by: Qijiang Fan <[email protected]>
Reviewed-by: Dan Erat <[email protected]>
Reviewed-by: Shuhei Takahashi <[email protected]>
diff --git a/docs/test_dependencies.md b/docs/test_dependencies.md
index 0ed87d4..a2c2b03 100644
--- a/docs/test_dependencies.md
+++ b/docs/test_dependencies.md
@@ -15,6 +15,8 @@
 *   `chrome_login` - Implies `chrome` with the further requirement that user
     login (i.e. using `session_manager` and `cryptohome`) is supported.
 *   `display_backlight` - An internal display backlight.
+*   `selinux` - An SELinux-enabled board. All Android boards are
+    SELinux-enabled.
 *   `tpm` - A [Trusted Platform Module] chip.
 *   `vm_host` - The ability to [run virtual machines].