[CSS Grid Layout] Grid items must set a new formatting context.

According to the spec, "A grid item establishes a new formatting context
for its contents." Hence, margins should not collapse even when they may
be adjoining to its content's margins. It also prevents any 'float' protruding
content on the adjoining grid items.

BUG=430100

Review URL: https://codereview.chromium.org/697653004

git-svn-id: svn://svn.chromium.org/blink/trunk@185214 bbb929c8-8fbe-4397-9dbb-9b2b20218538