Add function to document_loader to get PDF as pp::FileRef

pdf/pdf.gyp

 {
   'variables': {
     'chromium_code': 1,
     'pdf_engine%': 0,  # 0 PDFium
   },
   'target_defaults': {
     'cflags': [
       '-fPIC',
     ],
   },
@@ skipping 23 matching lines @@
         'control.h',
         'control.cc',
         'document_loader.h',
         'document_loader.cc',
         'draw_utils.cc',
         'draw_utils.h',
         'fading_control.cc',
         'fading_control.h',
         'fading_controls.cc',
         'fading_controls.h',
+        'file_writer.cc',
+        'file_writer.h',
         'instance.cc',
         'instance.h',
         'number_image_generator.cc',
         'number_image_generator.h',
         'out_of_process_instance.cc',
         'out_of_process_instance.h',
         'page_indicator.cc',
         'page_indicator.h',
         'paint_aggregator.cc',
         'paint_aggregator.h',
@@ skipping 106 matching lines @@
                 'pdf',
                 '../breakpad/breakpad.gyp:dump_syms',
               ],
             }],
           ],
         },
       ],
     },],  # OS=="linux" and chromeos==0
   ],
 }