Remove disabling of C4267 in 64-bit gn builds

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 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("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 817 matching lines @@
       # should work through these at some point -- they may be removed from
       # the RTM release in the /W4 set.
       "/wd4456",
       "/wd4457",
       "/wd4458",
       "/wd4459",
     ]
 
     if (visual_studio_version == "2015") {
       cflags += [
-        # VC++ 2015 changes 32-bit size_t truncation warnings from 4244 to 4267.
-        # Example: short TruncTest(size_t x) { return x; }
-        # Since we disable 4244 we need to disable 4267 during migration.
-        # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
-        "/wd4267",
-
         # C4312 is a VS 2015 64-bit warning for integer to larger pointer.
         # TODO(brucedawson): fix warnings, crbug.com/554200
         "/wd4312",
 
         # TODO(brucedawson): http://crbug.com/593448 - C4595 is an 'illegal
         # inline operator new' warning that is new in VS 2015 Update 2.
         # This is equivalent to clang's no-inline-new-delete warning.
         # See http://bugs.icu-project.org/trac/ticket/11122
         "/wd4595",
       ]
+
+      if (current_cpu == "x86") {
+        cflags += [
+          # VC++ 2015 changes 32-bit size_t truncation warnings from 4244 to
+          # 4267. Example: short TruncTest(size_t x) { return x; }
+          # Since we disable 4244 we need to disable 4267 during migration.
+          # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
+          "/wd4267",
+        ]
+      }
     }
 
     # VS xtree header file needs to be patched or 4702 (unreachable code
     # warning) is reported if _HAS_EXCEPTIONS=0. Disable the warning if xtree is
     # not patched.
     if (!msvs_xtree_patched &&
         exec_script("../../win_is_xtree_patched.py", [], "value") == 0) {
       cflags += [ "/wd4702" ]  # Unreachable code.
     }
 
@@ skipping 536 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }