Remove hasFallbackContent from RenderEmbeddedObject.

This belongs on the HTML elements, not on the renderer which is torn
down when the fallback content is attached.

BUG=379594

Review URL: https://codereview.chromium.org/301353005

git-svn-id: svn://svn.chromium.org/blink/trunk@175239 bbb929c8-8fbe-4397-9dbb-9b2b20218538