Empty inline elements always get a line box.

CSS 2.1 (http://www.w3.org/TR/CSS2/visudet.html#leading) says: "Empty inline 
elements generate empty inline boxes, but these boxes still have margins, padding, 
borders and a line height, and thus influence [line-height calculation] just 
like elements with content."

We weren't creating lineboxes for empty inline elements when they occurred
between a single trailing space and a single leading space - this is because
our linebox creation was based on the too-narrow premise that we were already
ignoring spaces. However, if an empty inline element comes after a trailing 
space but before one or more leading spaces then it will be ignored as part 
of the collapsed space between the trailing space and the next non-collapsible 
space - so we should force the creation of a linebox in such cases.

This allows us to pass Hixie's empty inline test: 
http://www.hixie.ch/tests/evil/mixed/emptyinline.html

BUG=328939

Review URL: https://codereview.chromium.org/300853007

git-svn-id: svn://svn.chromium.org/blink/trunk@175205 bbb929c8-8fbe-4397-9dbb-9b2b20218538