Allow HTMLPlugInElement to host UA shadow DOM.

When in "use placeholder content" mode, the element will
create an ordinary RenderBlockFlow as its renderer, and
so will be laid out as a block or inline-block.

A rule was added to StyleAdjuster to ensure that the
default width/height rules for replaced elements are
applied in this case. Since plugins have no intrinsic
ratio, this should be equivalent to mapping 'auto' to
the appropriate defaults.

Finally, a hook in Internals was added to allow layout
tests to trigger this mode, and two layout tests using
this were added.

BUG=364716

Review URL: https://codereview.chromium.org/369893006

git-svn-id: svn://svn.chromium.org/blink/trunk@180487 bbb929c8-8fbe-4397-9dbb-9b2b20218538