Fix building pdfium for x64 with ninja under windows.

build/standalone.gypi

 # Copyright 2014 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Definitions to be used when building stand-alone PDFium binaries.
 
 {
   'variables': {
     'component%': 'static_library',
     'clang%': 0,
@@ skipping 113 matching lines @@
               '-Os',
             ],
             'cflags': [
               '-fdata-sections',
               '-ffunction-sections',
               '-O2',
             ],
           }],
         ],  # conditions
       },
+      'Debug_x64': {
+        'inherit_from': ['Debug'],
+        'msvs_configuration_platform': 'x64',
+      },
+      'Release_x64': {
+        'inherit_from': ['Release'],
+        'msvs_configuration_platform': 'x64',
+      },
     },
     'defines!': [
       'DEBUG',
     ],
     'cflags': [
       '-Wall',
       '-W',
       '-Wno-unused-parameter',
       '-pthread', '-fno-exceptions',
       '-fvisibility=hidden',
@@ skipping 101 matching lines @@
           }],
         ],  # target_conditions
       },  # target_defaults
     }],  # OS=="mac"
   ],
   'xcode_settings': {
     # See comment in Chromium's common.gypi for why this is needed.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   }
 }