GuestView: Rename extension_id to embedder_extension_id

GuestViews will be able to host chrome extension pages shortly. This small
change clarifies that a GuestView takes in an embedder_extension_id rather than
the embedded content's extension ID. This CL also allows for empty extension IDs.

BUG=293732

Review URL: https://codereview.chromium.org/184973005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254908 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254908