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/wifi_provider.cc b/wifi_provider.cc
index f2fd08b..34fc88b 100644
--- a/wifi_provider.cc
+++ b/wifi_provider.cc
@@ -70,7 +70,8 @@
       manager_(manager),
       running_(false),
       total_frequency_connections_(-1L),
-      time_(Time::GetInstance()) {}
+      time_(Time::GetInstance()),
+      disable_vht_(false) {}
 
 WiFiProvider::~WiFiProvider() {}