Make types stricter for MediaDocument and make it AttachLazily

This the MediaDocument counterpart to r153834 where I cleaned up the types and made
ImageDocument lazyAttach its elements. This should make all the method calls in
MediaDocumentParser::createDocumentStructure non-virtual as well.

R=abarth@chromium.org,ojan@chromium.org,eseidel@chromium.org

Review URL: https://chromiumcodereview.appspot.com/18313008

git-svn-id: svn://svn.chromium.org/blink/trunk@154097 bbb929c8-8fbe-4397-9dbb-9b2b20218538