Default silent to platform convention

Tests: WebKit/WebKit#13912.

Closes #193.

Author: marcoscaceres

notifications.bs

@@ -83,8 +83,9 @@ initially false. When true, indicates that the end user should be alerted after
 an existing notification.
 
 <p>A <a>notification</a> has an associated
-<dfn for=notification id=silent-preference-flag>silent preference</dfn> boolean, which is initially
-false. When true, indicates that no sounds or vibrations should be made.
+<dfn for=notification id=silent-preference-flag>silent preference</dfn> boolean or null, which is
+initially null. When true, indicates that no sounds or vibrations should be made. When null,
+indicates that producing sounds or vibrations should be left to platform conventions.
 
 <p>A <a>notification</a> has an associated
 <dfn for=notification id=require-interaction-preference-flag>require interaction preference</dfn>
@@ -616,7 +617,7 @@ interface Notification : EventTarget {
   [SameObject] readonly attribute FrozenArray&lt;unsigned long> vibrate;
   readonly attribute EpochTimeStamp timestamp;
   readonly attribute boolean renotify;
-  readonly attribute boolean silent;
+  readonly attribute boolean? silent;
   readonly attribute boolean requireInteraction;
   [SameObject] readonly attribute any data;
   [SameObject] readonly attribute FrozenArray&lt;NotificationAction> actions;
@@ -635,7 +636,7 @@ dictionary NotificationOptions {
   VibratePattern vibrate;
   EpochTimeStamp timestamp;
   boolean renotify = false;
-  boolean silent = false;
+  boolean? silent = null;
   boolean requireInteraction = false;
   any data = null;
   sequence&lt;NotificationAction> actions = [];