Add a custom element to own structure of shadow DOM plugin placeholders.

This creates PluginPlaceholderElement, which manages a subtree of
elements which represent plugin placeholder content and exposes an
interface to manipulate this content. It is expected that this interface
will grow to encompass more than a single string in the future.

This change also adds an Internals method which uses this, so that its
appearance can be covered by layout tests, and adds one such layout test.

Ultimately, this class will be manipulated by a glue class which controls
this content on behalf of the embedder.

BUG=364716

Review URL: https://codereview.chromium.org/522783002

git-svn-id: svn://svn.chromium.org/blink/trunk@182014 bbb929c8-8fbe-4397-9dbb-9b2b20218538