[PPAPI] ResourceVar now holds a pending renderer and browser host ID.

It is likely that resources will be backed by a host in the renderer or
browser, or both. Therefore, HostResourceVar should provide a consistent
way to tell the plugin the pending host IDs.

BUG=177017

Review URL: https://codereview.chromium.org/24196005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225380 0039d316-1c4b-4281-b951-d872f2087c98