Enable policy test cases for VirtualKeyboardFeatures in chromeos

Now the master branch becomes M86, the test cases can be enabled.
(Previously, it always failed.)

Change-Id: I914a95b79c08ef7064c5563b20a018cb02541ece
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2279115Reviewed-by: Sergey Poromov <poromov@chromium.org>
Commit-Queue: Anqing Zhao <anqing@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784899}

chrome/test/data/policy/policy_test_cases.json

@@ -4253,7 +4253,7 @@
   },
 
   "VirtualKeyboardFeatures": {
-    "os": [],
+    "os": ["chromeos"],
     "policy_pref_mapping_test": [
       {
         "policies": {