Store opener information in embedder layer.

This is analogous to the transformation we did for frame tree. It 
also resolves several crashes when accessing the opener from the
the embedder if the opener frame had already been detached but 
not yet destroyed.

BUG=335947

Review URL: https://codereview.chromium.org/174453004

git-svn-id: svn://svn.chromium.org/blink/trunk@167878 bbb929c8-8fbe-4397-9dbb-9b2b20218538