[VK] Whitelist accessibility keyboard pref for Tast tests.

We would like to test the accessibility keyboard in Tast, but it's
activated based on a pref. We'll whitelist this pref so that Tast
can set it to true to bring up the accessibility keyboard.

Note: this introduces another ash dependency from chromeos.

Bug: 879073
Change-Id: I593e4d544ea4f0e1717c4a683d0fe65d2a7a7a38
Reviewed-on: https://chromium-review.googlesource.com/c/1485691Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635254}

chrome/browser/chromeos/extensions/autotest_private/autotest_private_api.cc

@@ -8,6 +8,7 @@
 #include <sstream>
 #include <utility>
 
+#include "ash/public/cpp/ash_pref_names.h"
 #include "ash/public/interfaces/ash_message_center_controller.mojom.h"
 #include "ash/public/interfaces/constants.mojom.h"
 #include "ash/shell.h"
@@ -181,6 +182,8 @@ std::string SetWhitelistedPref(Profile* profile,
         ash::mojom::AssistantAllowedState::ALLOWED) {
       return "Assistant is not available for the current user";
     }
+  } else if (pref_name == ash::prefs::kAccessibilityVirtualKeyboardEnabled) {
+    DCHECK(value.is_bool());
   } else {
     return "The pref " + pref_name + "is not whitelisted.";
   }