Allow content property to roundtrip

Before this patch the content property parsing did
not allow comma separated values but did use comma
separated value lists for serialization. This patch changes
that behavior to always use space separated lists and
adds a test to make sure roundtripping content works.

BUG=428445

Review URL: https://codereview.chromium.org/707923003

git-svn-id: svn://svn.chromium.org/blink/trunk@184993 bbb929c8-8fbe-4397-9dbb-9b2b20218538