RenderMedia should not allow generated content

When I removed the explicit treeScope() checks from HTMLMediaElement I accidentally enabled
::before and ::after on <video> and <audio> since they both override canHaveChildren to return
true to allow ShadowRoots. Unfortunately they also assume that the only renderer children they
ever have are from the shadow controls element not from generated content so they'll crash
in RenderMedia::layout.

To fix this I disabled generated content on RenderMedia so that nothing can shove a ::before or
::after down into it. Eventually we might consider allowing video::before and ::after which would
require modifying RenderMedia::layout to be generated content aware.

BUG=263810

Review URL: https://chromiumcodereview.appspot.com/20400002

git-svn-id: svn://svn.chromium.org/blink/trunk@154979 bbb929c8-8fbe-4397-9dbb-9b2b20218538