Mac: Fix fallback font selection for custom fonts

This fixes a bug where a bullet character (U+25CF) becomes tofu when it
is a fallback for a webfont and italic style. The fallback font
(Helvetica) has a glyph for U+25CF, but its italic variation (Helvetica
Oblique) does not. In such a case, we should use synthetic oblique.

BUG=373389
TEST=fast/text/fallback-for-custom-font.html

Review URL: https://codereview.chromium.org/300393003

git-svn-id: svn://svn.chromium.org/blink/trunk@175243 bbb929c8-8fbe-4397-9dbb-9b2b20218538