Port https://codereview.chromium.org/398163006 to gn.

BUILD.gn

 # Copyright 2014 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.
 
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_config") {
   cflags = []
   include_dirs = [ "third_party/freetype/include" ]
@@ skipping 20 matching lines @@
     }
   }
 
   if (is_win) {
     cflags += [
       "/wd4005",
       "/wd4018",
       "/wd4146",
       "/wd4333",
       "/wd4345",
+      "/wd4267",
       # TODO(thestig): Fix all instances, remove this, pdfium:29
       "/wd4245",
       "/wd4310",
       "/wd4389",
       "/wd4701",
       "/wd4702",
       "/wd4706",
       "/wd4800",
     ]
   }
@@ skipping 713 matching lines @@
       "//v8",
       "//v8:v8_libplatform",
     ]
     include_dirs += [
       "//v8",
       "//v8/include",
     ]
   }
   configs += [ ":pdfium_config" ]
 }