Bug 1583052 - Fix a check which really wants composed doc semantics, not uncomposed. r=smaug

I don't know why it'd want uncomposed doc semantics.

Differential Revision: https://phabricator.services.mozilla.com/D46775

dom/html/HTMLMediaElement.cpp

@@ -4265,9 +4265,7 @@ nsresult HTMLMediaElement::BindToTree(BindContext& aContext, nsINode& aParent) {
     // Construct Shadow Root so web content can be hidden in the DOM.
     AttachAndSetUAShadowRoot();
     NotifyUAWidgetSetupOrChange();
-  }
 
-  if (IsInUncomposedDoc()) {
     // The preload action depends on the value of the autoplay attribute.
     // It's value may have changed, so update it.
     UpdatePreloadAction();