<webview>: guest pages embedded in WebUI should appear in chrome://inspect

BUG=332410
TBR=kaznacheev@chromium.org

Review URL: https://codereview.chromium.org/125593006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243966 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243966