Avoid loading unnecessary font faces as primary font

Currently Blink always loads font face with its unicode-range contains
space character (U+20), even if the document has no ASCII character of
that font, because the font face with space is used as the primary font
of the segmented font.

To avoid such unnecessary loads, this patch changes the primary font
selection rule for segmented fonts. When the font face with space is not
yet loaded, use the first loaded face in the SegmentedFontFace (if any)
as the primary font.

BUG=379932
TEST=fast/css/font-face-unicode-range-load.html

Review URL: https://codereview.chromium.org/323003005

git-svn-id: svn://svn.chromium.org/blink/trunk@176963 bbb929c8-8fbe-4397-9dbb-9b2b20218538