Ensure we compute the height of replaced elements to 'auto' when appropriate.

If a replaced element has a percentage height specified then its height should
compute to 'auto' when its containing block does not have a height 'specified
explicitly'. We were taking this to mean when its containing block's specified
height value is 'auto' - in fact it means when the containing block's computed
height is auto. This brings us into line with IE and Firefox.

http://lists.w3.org/Archives/Public/www-style/2014Jun/0079.html

BUG=385877

Review URL: https://codereview.chromium.org/347053007

git-svn-id: svn://svn.chromium.org/blink/trunk@177143 bbb929c8-8fbe-4397-9dbb-9b2b20218538