Clear the media element's autoplaying flag when autoplaying

The problem was pointed out by Frank Liberato:
https://codereview.chromium.org/1470153004/diff/40001/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp#newcode2976

"letting a video element with an autoplay attribute play to completion,
then seeking to the middle, starts playback immediately when updating
the network state. that seems really weird."

source

@@ -31484,6 +31484,8 @@ interface <dfn>HTMLMediaElement</dfn> : <span>HTMLElement</span> {
        <li><span>Queue a task</span> to <span>fire a simple event</span> named <code
        data-x="event-media-playing">playing</code> at the element.</li>
 
+       <li>Set the <span>autoplaying flag</span> to false.</li>
+
       </ol>
 
       <p class="note">User agents do not need to support autoplay, and it is suggested that user
@@ -118108,6 +118110,7 @@ INSERT INTERFACES HERE
   Francis Brosnan Blazquez,
   Franck 'Shift' Qu&eacute;lain,
   Frank Barchard,
+  Frank Liberato,
   Fredrik S&ouml;derquist,
   &#40284;&#39164;&#25991;&#25935; (Fumitoshi Ukai),
   Futomi Hatano,