Implement font-family: initial.

It appears that this never worked correctly. To implement,
I extracted the code we use to set initial value for document and
started using it when applying initial font-family value.

BUG=354110
R=eseidel,eae

Review URL: https://codereview.chromium.org/197213034

git-svn-id: svn://svn.chromium.org/blink/trunk@169592 bbb929c8-8fbe-4397-9dbb-9b2b20218538