Fix broken site navigation when using Safari on Mac OS X

I tracked this down to two lines which referred to style.position.left and style.position.top, which should be style.left and style.top. Safari seems to have a habit of failing silently in these situations, which resulted in the navigation breaking.

I've tested on Safari on iPad, which was exhibiting the same bug.

Fixes https://crosswalk-project.org/jira/browse/XWALK-903