HTMLMediaElement should use didNotifySubtreeInsertionsToDocument()

HTMLMediaElement::insertedInto calls configureMediaControls() which then
goes and creates the media controls and inserts them into the ShadowRoot.
This would hit the DOM mutation assert I plan to add when I remove
ChildNodesLazySnapshot, instead we need to use
didNotifySubtreeInsertionsToDocument which is a safe place to mutate the
DOM in response to being inserted.

Review URL: https://codereview.chromium.org/231733006

git-svn-id: svn://svn.chromium.org/blink/trunk@171209 bbb929c8-8fbe-4397-9dbb-9b2b20218538