Remove TreatNullAs=NullString for HTMLAreaElement

http://whatwg.org/html#htmlareaelement

The tests for the updated properties pass in Firefox Nightly, IE11
Release Preview and Opera 12.16, with these exceptions:

Firefox reflects ping as a URL property, which is not per spec. 

IE11 reflects coords as '0,0,0,0' and shape as 'rect', while it doesn't
support ping.

Presto doesn't support ping.

BUG=310298

Review URL: https://codereview.chromium.org/59383002

git-svn-id: svn://svn.chromium.org/blink/trunk@161340 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Replace hard-coded pixel sized adjustments of overview items in CSS with flex, so we can play with overview size.

BUG=305863
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/59863002

git-svn-id: svn://svn.chromium.org/blink/trunk@161341 bbb929c8-8fbe-4397-9dbb-9b2b20218538