Set some MessageEvent's isTrusted attributes to true

Closes #1602. Browsers already behave this way, and it is more
consistent with how the rest of the spec almost always fires trusted
events. See also #1912.

In terms of spec mechanics, this is done by using the new "create an
event" hook from DOM, which sets isTrusted to true for us.

source

@@ -55409,7 +55409,7 @@ fur
   data-x="concept-form-reset">reset</dfn>, run these steps:</p>
 
   <ol>
-   <li><p>Let <var>isTrusted</var> false, if this algorithm is invoked by the <code
+   <li><p>Let <var>isTrusted</var> be false, if this algorithm is invoked by the <code
    data-x="dom-form-reset">reset()</code> method, and true otherwise.</p></li>
 
    <li><p>Let <var>reset</var> be the result of <span data-x="concept-event-fire">firing an
@@ -93076,13 +93076,15 @@ any-char      = %x0000-0009 / %x000B-000C / %x000E-10FFFF
    <li><p>If the <var>data</var> buffer's last character is a U+000A LINE FEED (LF)
    character, then remove the last character from the <var>data</var> buffer.</p></li>
 
-   <li><p>Let <var>event</var> be a new <code>MessageEvent</code> object, with its <code
-   data-x="dom-Event-type">type</code> attribute initialized to <code
-   data-x="event-message">message</code>, <code data-x="dom-MessageEvent-data">data</code> attribute
-   initialized to <var>data</var>, <code data-x="dom-MessageEvent-origin">origin</code> attribute
-   initialized to the <span data-x="Unicode serialization of an origin">Unicode serialization</span>
+   <li><p>Let <var>event</var> be the result of <span>creating an event</span> using
+   <code>MessageEvent</code>.</p></li>
+
+   <li><p>Initialize <var>event</var>'s <code data-x="dom-Event-type">type</code> attribute to <code
+   data-x="event-message">message</code>, its <code data-x="dom-MessageEvent-data">data</code>
+   attribute to <var>data</var>, its <code data-x="dom-MessageEvent-origin">origin</code> attribute
+   to the <span data-x="Unicode serialization of an origin">Unicode serialization</span>
    of the <span>origin</span> of the event stream's final URL (i.e. the URL after redirects), and
-   <code data-x="dom-MessageEvent-lastEventId">lastEventId</code> attribute initialized to the <span
+   its <code data-x="dom-MessageEvent-lastEventId">lastEventId</code> attribute to the <span
    data-x="concept-event-stream-last-event-id">last event ID string</span> of the event
    source.</p></li>
 
@@ -94817,11 +94819,13 @@ interface <dfn>MessagePort</dfn> : <span>EventTarget</span> {
    algorithm runs scripts). We don't throw an exception for 'doomed' being true, because this can't
    necessarily be detected right now every time -->
 
-   <li><p>Let <var>e</var> be a new <code>MessageEvent</code> object, with its <code
-   data-x="dom-Event-type">type</code> attribute initialized to <code
-   data-x="event-message">message</code>, <code data-x="dom-MessageEvent-data">data</code> attribute
-   initialized to <var>messageClone</var>, and <code data-x="dom-MessageEvent-ports">ports</code>
-   attribute initialized to <var>newPorts</var>.</p></li>
+   <li><p>Let <var>e</var> be the result of <span>creating an event</span> using
+   <code>MessageEvent</code>.</p></li>
+
+   <li><p>Initialize <var>e</var>'s <code data-x="dom-Event-type">type</code> attribute to <code
+   data-x="event-message">message</code>, its <code data-x="dom-MessageEvent-data">data</code>
+   attribute to <var>messageClone</var>, and its <code data-x="dom-MessageEvent-ports">ports</code>
+   attribute to <var>newPorts</var>.</p></li>
 
    <li>
     <p>Add a <span data-x="concept-task">task</span> that runs the following steps to the <span>port