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

ui/gfx/platform_font_mac.h

Index: ui/gfx/platform_font_mac.h
diff --git a/ui/gfx/platform_font_mac.h b/ui/gfx/platform_font_mac.h
index 5b2f5beed50f9c36e41d19890235099da5912426..e127fa12dcb4f53708fbd685c21e1bdf817281c7 100644
--- a/ui/gfx/platform_font_mac.h
+++ b/ui/gfx/platform_font_mac.h
@@ -55,8 +55,8 @@ class PlatformFontMac : public PlatformFont {
 
   // The NSFont instance for this object. If this object was constructed from an
   // NSFont instance, this holds that NSFont instance. Otherwise this NSFont
-  // instance is constructed from the name, size, and style, and if there is no
-  // active font that matched those criteria, this object may be nil.
+  // instance is constructed from the name, size, and style. If there is no
+  // active font that matched those criteria a default font is used.
   base::scoped_nsobject<NSFont> native_font_;
 
   // The name/size/style trio that specify the font. Initialized in the