Fix building pdfium for x64 with ninja under windows.

This adds the necessary directives to the standalone gyp file.

R=jschuh@chromium.org, jam@chromium.org
BUG=22

Patch from Michael Doppler <m.doppler@gmail.com>.

Review URL: https://codereview.chromium.org/360273002

build/standalone.gypi

diff --git a/build/standalone.gypi b/build/standalone.gypi
index 967db1e..5a65fa5 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -131,6 +131,14 @@
           }],
         ],  # conditions
       },
+      'Debug_x64': {
+        'inherit_from': ['Debug'],
+        'msvs_configuration_platform': 'x64',
+      },
+      'Release_x64': {
+        'inherit_from': ['Release'],
+        'msvs_configuration_platform': 'x64',
+      },
     },
     'defines!': [
       'DEBUG',