Make HTMLMediaElement destructor not to dispatch load event

This patch changes HTMLMediaElement destructor not to dispatch load event to make JavaScript load event handler in stable state, e.g. accessing destructing media elements in event handler.

Original crash bug is occurred in EditorClientImpl::clearUndoRedoOperations(), which is called recursively during undo stack destruction. The first call of  EditorClientImpl::clearUndoRedoOperations() is kicked by removing IFRAME via HTMLFrameOwnerElement::OwnerElement(). Second call is kicked by destructing HTMLMediaElement in undo stack via ResourceFetcher::didLoadResource() in media player, associated to HTMLMediaElement, destruction.

BUG=275223
TEST=LayoutTests/editing/undo/audio-in-undo-stack-crash.html

Review URL: https://chromiumcodereview.appspot.com/23555002

git-svn-id: svn://svn.chromium.org/blink/trunk@157865 bbb929c8-8fbe-4397-9dbb-9b2b20218538