[CSS Grid Layout] Fix gridDefinition* style dumping

The issue was that we abused the style system when parsing and thus
would confuse element.style.gridDefinition{rows|columns} when trying
to dump it back. An example would be that minmax() was stored as a
CSSValueList, not as a CSSFunctionValue.

This change aligns the internal storage after parsing with what is
parsed. This fixes element.style as the CSSValues match what was
passed by the author.

The existing tests for grid-definition-* parsing were extended to
cover element.style when it made sense ('none' is returned as ''
which makes little sense to test).

BUG=234208

Review URL: https://chromiumcodereview.appspot.com/22926017

git-svn-id: svn://svn.chromium.org/blink/trunk@156611 bbb929c8-8fbe-4397-9dbb-9b2b20218538