shill: WiFi: Set disable_vht parameter

Provide a run-time Manager property which sets the WiFi
provider's "disable_vht" field.  This will be used to enable or
disable VHT in wpa_supplicant.

CQ-DEPEND=CL:189960,CL:189941
BUG=chromium:352260
TEST=Unit tests

Change-Id: Id380ce15982418dea2845f722460c612aee565db
Reviewed-on: https://chromium-review.googlesource.com/189961
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
diff --git a/wpa_supplicant.h b/wpa_supplicant.h
index 7736bd1..99da80c 100644
--- a/wpa_supplicant.h
+++ b/wpa_supplicant.h
@@ -79,6 +79,7 @@
   static const char kNetworkModeAccessPoint[];
   static const char kNetworkPropertyBgscan[];
   static const char kNetworkPropertyCaPath[];
+  static const char kNetworkPropertyDisableVHT[];
   static const char kNetworkPropertyEapKeyManagement[];
   static const char kNetworkPropertyEapIdentity[];
   static const char kNetworkPropertyEapEap[];