Center images when loading image urls in the main frame.

Do the same as Firefox and Presto when displaying raw image urls in the
main frame: center them in the view.

If this works out OK we should apply it to images loaded in iframes too.

BUG=27987

Review URL: https://codereview.chromium.org/247293005

git-svn-id: svn://svn.chromium.org/blink/trunk@172169 bbb929c8-8fbe-4397-9dbb-9b2b20218538