Fix for magnifying glass to magnify/shrink images when the zoom is not at 100%.

Allow zoom/restore in a zoomed image document. It is a regression from the earlier fix:
http://src.chromium.org/viewvc/blink?revision=156402&view=revision

BUG=324086
R=levi, tony

Review URL: https://codereview.chromium.org/262053005

git-svn-id: svn://svn.chromium.org/blink/trunk@176023 bbb929c8-8fbe-4397-9dbb-9b2b20218538