[CSSGridLayout] ASSERTION: Bad cast in parseGridTemplateRowsAndAreas

An empty list of grid line names (represented by "()") does not add anything to
the list of parsed values. That's why trying to concatenate an adjacent list of
grid line names was failing, because we were trying to concatenate a list with
the last parsed CSSValue which was not the expected grid line names list.

BUG=402004

Review URL: https://codereview.chromium.org/569563002

git-svn-id: svn://svn.chromium.org/blink/trunk@181915 bbb929c8-8fbe-4397-9dbb-9b2b20218538