Add last resort to fallback chain in FontFallbackList::primaryFontData

Change FontFallbackList::primaryFontData to fall back on the last
resort fallback font if fontDataAt returns NULL. This matches the
behavior of the getFontData method and ensures that a valid FontData
object is returned as expected by the primarySimpleFontData method.

BUG=344989
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/179983007

git-svn-id: svn://svn.chromium.org/blink/trunk@167862 bbb929c8-8fbe-4397-9dbb-9b2b20218538