[CSS Grid Layout] Named grid line span not allowed for automatic positions

From the grid layout spec we can extract the following sentence:
"If the grid item has an automatic position and a grid span for a named
line in a given dimension, instead treat the grid span as one."

The main reason is that as you don't know where the item is going to be
placed (because of it's auto-positioned), spanning to a particular line
makes no sense.

Updated GridResolvedPosition code to include this restriction when
resolving positions.

Adapted auto-placement algorithm implementation in RenderGrid as now
it's possible to pass row and column spans to
GridIterator::nextEmptyGridArea() simplifying the code.

Updated layout test to the new behavior.

BUG=353789
TEST=fast/css-grid-layout/grid-item-auto-placement-automatic-span.html

Review URL: https://codereview.chromium.org/320093002

git-svn-id: svn://svn.chromium.org/blink/trunk@176295 bbb929c8-8fbe-4397-9dbb-9b2b20218538