Ship navigator.vibrate behind featurePolicyExperimentalFeatures flag

Due to complexity between user gesture and vibrate, we decided to remove vibrate from V1 feature policy.

Review-Url: https://codereview.chromium.org/2801153002
Cr-Commit-Position: refs/heads/master@{#462893}

third_party/WebKit/Source/modules/vibration/NavigatorVibration.cpp

@@ -81,6 +81,7 @@ bool NavigatorVibration::vibrate(Navigator& navigator,
   // TODO(lunalu): When FeaturePolicy is ready, take out the check for the
   // runtime flag. Please pay attention to the user gesture code below.
   if (RuntimeEnabledFeatures::featurePolicyEnabled() &&
+      RuntimeEnabledFeatures::featurePolicyExperimentalFeaturesEnabled() &&
       !frame->isFeatureEnabled(blink::WebFeaturePolicyFeature::Vibrate)) {
     frame->domWindow()->printErrorMessage(
         "Navigator.vibrate() is not enabled in feature policy for this "