Positioning should account for the grid items' margins

This change just adds the margin before / start to the grid items'
positions. In order to get consistent behavior, we now have to call
findChildLogicalPosition after layout (that's where we compute the
margins).

Also removed some FIXMEs that were handled in the code.

Review URL: https://chromiumcodereview.appspot.com/23463036

git-svn-id: svn://svn.chromium.org/blink/trunk@157925 bbb929c8-8fbe-4397-9dbb-9b2b20218538