Fix webfont loading fallback height issue (first step)

When a webfont is loading, a temporary font is created which may be
used to calculate the layout height of the text and override the
correct fallback font. To prevent that, skip the temporary font in
FontFallbackList::primaryFontData().

FIXME: remaining issues:
1. When the webfont is the only font face in the CSS font-family,
the fallback font is still incorrect when the webfont is loading;
2. When the fallback font is monospace, when it is fell back to,
the default font-size doesn't match (16px vs 13px).

BUG=299010
TEST=http/tests/webfont/fallback-font-while-loading.html

Review URL: https://codereview.chromium.org/24895002

git-svn-id: svn://svn.chromium.org/blink/trunk@159065 bbb929c8-8fbe-4397-9dbb-9b2b20218538