<webview>: Enforce stronger GuestView member constness guarantees

BUG=none
TBR=fsamuel@chromium.org

Review URL: https://codereview.chromium.org/65443002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233723 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233723