Don't crash when using IME on an input field with a CSS-specified font.

content/common/mac/font_descriptor.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 CONTENT_COMMON_MAC_FONT_DESCRIPTOR_H_
 #define CONTENT_COMMON_MAC_FONT_DESCRIPTOR_H_
 
 #include "base/strings/string16.h"
 #include "content/common/content_export.h"
 
 #ifdef __OBJC__
 @class NSFont;
 #else
 class NSFont;
 #endif
 
 // Container to allow serializing an NSFont over IPC.
 struct CONTENT_EXPORT FontDescriptor {
   explicit FontDescriptor(NSFont* font);
   FontDescriptor(base::string16 name, float size);
 
   FontDescriptor() : font_point_size(0) {}
 
-  // Return an autoreleased NSFont corresponding to the font description.
+  // Return an autoreleased NSFont corresponding to the font description. Note
+  // that this will return nil if the font specified isn't available in this
+  // process.
   NSFont* ToNSFont() const;
 
   // Name of the font.
   base::string16 font_name;
 
   // Size in points.
   float font_point_size;
 };
 
 #endif  // CONTENT_COMMON_MAC_FONT_DESCRIPTOR_H_