[CSS Grid Layout] Use Render methods, rather than style, to
 determine shrink-to-fit behavior

In order to determine whether the RemainingSpace is 
undetermined or not (shrink-to-fit) the current algorithm 
is checking out the style for clarifying if the Render object 
is float or absolute positioned.

It's better to use the Render functions instead, since the 
style might be applied or not, depending on several factors, 
like CSS properties inheritance.

BUG=79180, 234204

Review URL: https://codereview.chromium.org/181333003

git-svn-id: svn://svn.chromium.org/blink/trunk@168573 bbb929c8-8fbe-4397-9dbb-9b2b20218538