PlatformFontMac: Use a default NSFont when an invalid font is passed.

ui/gfx/font_fallback_mac.mm

Index: ui/gfx/font_fallback_mac.mm
diff --git a/ui/gfx/font_fallback_mac.mm b/ui/gfx/font_fallback_mac.mm
index 6cfc7e36b8b713de6d9cfacfd347fcc0279bd98e..6a4d66f1d4083966dc287370e117db35e98bb5c8 100644
--- a/ui/gfx/font_fallback_mac.mm
+++ b/ui/gfx/font_fallback_mac.mm
@@ -40,6 +40,7 @@ bool FontsEqual(CTFontRef lhs, CTFontRef rhs) {
 }  // namespace
 
 std::vector<Font> GetFallbackFonts(const Font& font) {
+  DCHECK(font.GetNativeFont());
   // On Mac "There is a system default cascade list (which is polymorphic, based
   // on the user's language setting and current font)" - CoreText Programming
   // Guide.
@@ -51,6 +52,8 @@ std::vector<Font> GetFallbackFonts(const Font& font) {
           static_cast<CTFontRef>(font.GetNativeFont()), languages_cf));
 
   std::vector<Font> fallback_fonts;
+  if (!cascade_list)
+    return fallback_fonts;  // This should only happen for an invalid |font|.
 
   const CFIndex fallback_count = CFArrayGetCount(cascade_list);

[tapted, 2017/06/28 03:46:10]

Currently it's this line that crashes -- null can't be passed to CFArrayGetCount

   for (CFIndex i = 0; i < fallback_count; ++i) {