Disable warning 4267 in pdfium until it is fixed.

BUG=376399
TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/398163006

pdfium.gyp

diff --git a/pdfium.gyp b/pdfium.gyp
index 466e9c3..55a7db8 100644
--- a/pdfium.gyp
+++ b/pdfium.gyp
@@ -40,7 +40,7 @@
       }],
     ],
     'msvs_disabled_warnings': [
-      4005, 4018, 4146, 4333, 4345
+      4005, 4018, 4146, 4333, 4345, 4267
     ],
   },
   'targets': [