Disallow non-flexbox child renderers for RenderMedia

Inline children of replaced content is not handled and triggered the
ASSERT(curr->isRenderInline()) UNKNOWN in RenderInline::splitInlines().

The render tree at the point of failure was:

RenderView 0xd0c23404010                #document 0x2be215c04b48
  RenderBlock 0xd0c2341c010             HTML 0x2be215c101a8
    RenderBody 0xd0c2341c110            BODY 0x2be215c102b8
      RenderBlock (anonymous) 0xd0c2341c310
        RenderVideo 0xd0c23420010       VIDEO 0x2be215c48010
*         RenderInline (relative positioned) 0xd0c23424010 DIV 0x2be215c68010
      RenderBlock (anonymous) 0xd0c2341c210
      RenderBlock (anonymous) 0xd0c2341c410

Making ::-webkit-media-controls* internal would have fixed this:
https://codereview.chromium.org/662243003/
https://groups.google.com/a/chromium.org/d/msg/blink-dev/YCIaYPa_DhI/RyDlcHmD1_wJ

Unfortunately, that is not without risk. This fix allows
::-webkit-media-controls { display: none; } to keep working until the
usage can be measured.

BUG=415407, 426759

Review URL: https://codereview.chromium.org/689613002

git-svn-id: svn://svn.chromium.org/blink/trunk@184652 bbb929c8-8fbe-4397-9dbb-9b2b20218538