Fix getComputedStyle() for area element

The getComputedStyle() function didn't work as we expected for the area.
The reason is chrome set display area as none, this patch fix this problem.

This behavior matches Firefox.
The Internet Explorer 11 set area display as none.

BUG=392759

Review URL: https://codereview.chromium.org/442563002

git-svn-id: svn://svn.chromium.org/blink/trunk@180610 bbb929c8-8fbe-4397-9dbb-9b2b20218538