[CSS Grid Layout] Redefining grids inside media queries does not work

So the problem is that in
StyleBuilderFunctions::applyValueCSSPropertyGridTemplateAreas we were assuming
that we were always getting a fresh RenderStyle, something that is not true,
because we might have overlapping declarations (multipled matching media
queries, user styles, etc).

As that function might be called multiple times, we need to properly regenerate
the list of named grid lines each time the function is called because the
NamedGridLinesMap returned by RenderStyle contains both the old explicit named
grid lines and the old implicit named grid lines.

BUG=427481

Review URL: https://codereview.chromium.org/680063005

git-svn-id: svn://svn.chromium.org/blink/trunk@185045 bbb929c8-8fbe-4397-9dbb-9b2b20218538