Store a copy of generic font settings at CSSFontSelector.

A CSSFontSelector instance does not expect for generic font settings to
ever change within its lifetime. So, it makes sense to copy and stash
them locally and avoid holding/using a Document pointer for this
purpose.

Eventually, CSSFontSelector hopes to become a per-Document font context,
which knows nothing about the Document itself, but provides an API
that a Document could use.

This also fixes bug 236298, since determining generic font no longer relies on CSSFontSelector::m_document.

BUG=324991,236298
R=eae,eseidel,ksakamoto@chromium.org

Review URL: https://codereview.chromium.org/93783005

git-svn-id: svn://svn.chromium.org/blink/trunk@163130 bbb929c8-8fbe-4397-9dbb-9b2b20218538