Refactor GuestView to take in extension_id on construction

This is a prerequisite to support relative URL resolution to chrome-extension URLs.

The guest site instance is constructed in BrowserPluginGuestManager::CreateGuest with the appropriate host corresponding to the extension ID of the app/extension hosting the GuestView. This CL gets rid of some code duplication.

BUG=308114
R=creis@chromium.org, jam@chromium.org

Review URL: https://codereview.chromium.org/45313014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232144 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232144