Clarify that the custom margin value is not validated and add tests.

printing/print_settings.h

 // Copyright (c) 2011 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 PRINTING_PRINT_SETTINGS_H_
 #define PRINTING_PRINT_SETTINGS_H_
 
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
@@ skipping 23 matching lines @@
   ~PrintSettings();
 
   // Reinitialize the settings to the default values.
   void Clear();
 
   // Set printer printable area in in device units.
   void SetPrinterPrintableArea(gfx::Size const& physical_size_device_units,
                                gfx::Rect const& printable_area_device_units,
                                int units_per_inch);
 
-  void SetCustomMargins(const PageMargins& margins_in_points);
+  void SetCustomMargins(const PageMargins& requested_margins_in_points);
 
   // Equality operator.
   // NOTE: printer_name is NOT tested for equality since it doesn't affect the
   // output.
   bool Equals(const PrintSettings& rhs) const;
 
   void set_landscape(bool landscape) { landscape_ = landscape; }
   void set_printer_name(const string16& printer_name) {
     printer_name_ = printer_name;
   }
@@ skipping 77 matching lines @@
 
   // Printer's device effective dots per inch in both axis.
   int dpi_;
 
   // Is the orientation landscape or portrait.
   bool landscape_;
 
   // True if this printer supports AlphaBlend.
   bool supports_alpha_blend_;
 
-  // If margin type is custom, these are the margins.
-  PageMargins custom_margins_in_points_;
+  // If margin type is custom, this is what was requested.
+  PageMargins requested_custom_margins_in_points_;
 };
 
 }  // namespace printing
 
 #endif  // PRINTING_PRINT_SETTINGS_H_