Use the /WX flag to have link warnings treated as errors.

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 815 matching lines @@
 }
 
 # default_warnings ------------------------------------------------------------
 #
 # Collects all warning flags that are used by default.  This is used as a
 # subconfig of both chromium_code and no_chromium_code.  This way these
 # flags are guaranteed to appear on the compile command line after -Wall.
 config("default_warnings") {
   cflags = []
   cflags_cc = []
+  ldflags = []
 
   if (is_win) {
     if (treat_warnings_as_errors) {
       cflags += [ "/WX" ]
     }
+    if (fatal_linker_warnings) {
+      ldflags += [ "/WX" ]
+    }
 
     cflags += [
       # Assume UTF-8 by default to avoid code page dependencies.
       "/utf-8",
     ]
 
     cflags += [
       # Warnings permanently disabled:
 
       # C4091: 'typedef ': ignored on left of 'X' when no variable is
@@ skipping 798 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
   }
 }