Unloaded font segment should not affect primaryFontData

This patch fixes a bug where SVG fonts are not rendered correctly when
SVG font is part of a segmented font face that has unloaded font faces.
SegmentedFontData::isLoadingFallback() returns true for such segmented
font face, so FontFallbackList::primaryFontData() returned next
available font in the fallback list.

After this patch, FontFallbackList::primaryFontData() returns the first
font data with space character loaded.

BUG=369633
TEST=svg/custom/svg-fonts-unloaded-segment.html

Review URL: https://codereview.chromium.org/271633002

git-svn-id: svn://svn.chromium.org/blink/trunk@173859 bbb929c8-8fbe-4397-9dbb-9b2b20218538