Re-enable warning 4701 for GN (re-land)

BUILD.gn

 # Copyright 2016 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_overrides/v8.gni")
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_config") {
   cflags = []
@@ skipping 26 matching lines @@
       defines += [ "_FX_CPU_=_FX_X64_" ]
       cflags += [ "-fPIC" ]
     } else if (current_cpu == "x86") {
       defines += [ "_FX_CPU_=_FX_X86_" ]
     }
   }
 
   if (is_win) {
     cflags += [
       "/wd4267",
-      "/wd4701",
     ]
   }
 
   if (is_clang || is_posix) {
     # TODO(thestig): Fix all instances, remove this, pdfium:29
     cflags += [ "-Wno-sign-compare" ]
   }
 
   if (is_posix && !is_clang) {  # When GCC.
     cflags += [ "-Wno-error=strict-overflow" ]
@@ skipping 1656 matching lines @@
       "//v8:v8_libplatform",
     ]
     include_dirs += [
       "//v8",
       "//v8/include",
     ]
     configs += [ "//v8:external_startup_data" ]
   }
   configs += [ ":pdfium_config" ]
 }