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

ui/gfx/platform_font_mac.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_GFX_PLATFORM_FONT_MAC_H_
 #define UI_GFX_PLATFORM_FONT_MAC_H_
 
 #include "base/compiler_specific.h"
 #include "base/mac/scoped_nsobject.h"
 #include "base/macros.h"
@@ skipping 37 matching lines @@
                   int font_style,
                   Font::Weight font_weight);
 
   ~PlatformFontMac() override;
 
   // Calculates and caches the font metrics and inits |render_params_|.
   void CalculateMetricsAndInitRenderParams();
 
   // 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
   // constructors.
   const std::string font_name_;  // Corresponds to -[NSFont fontFamily].
   const int font_size_;
   const int font_style_;
   const Font::Weight font_weight_;
 
   // Cached metrics, generated in CalculateMetrics().
   int height_;
   int ascent_;
   int cap_height_;
 
   // Cached average width, generated in GetExpectedTextWidth().
   float average_width_ = 0.0;
 
   // Details about how the font should be rendered.
   FontRenderParams render_params_;
 
   DISALLOW_COPY_AND_ASSIGN(PlatformFontMac);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_PLATFORM_FONT_MAC_H_