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

build/config/win/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/clang/clang.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/win/visual_studio_version.gni")
 import("//build/toolchain/toolchain.gni")
 
@@ skipping 84 matching lines @@
   if (is_syzyasan) {
     assert(!is_win_fastlink)
     ldflags = [ "/PROFILE" ]
   } else {
     if (!is_debug && !is_component_build) {
       if (is_win_fastlink || is_clang) {
         # /PROFILE implies the following linker flags. Therefore if we are
         # skipping /PROFILE because it is incompatible with /DEBUG:FASTLINK
         # we should explicitly add these flags in order to avoid unintended
         # consequences such as larger binaries.
-        ldflags = [
+        ldflags += [

[brucedawson, 2016/12/21 22:22:53]

This is wrong. "+=" should only be allowed if ldflags was set earlier - the
conditional setting of ldflags at line 97 doesn't count.

TL;DR - this needs to be "=" instead of "+=".

[Wez, 2016/12/22 00:03:08]

Done.

           "/OPT:REF",
           "/OPT:ICF",
           "/INCREMENTAL:NO",
           "/FIXED:NO",
         ]
       } else {
         ldflags = [ "/PROFILE" ]
       }
     }
   }
@@ skipping 89 matching lines @@
 config("winver") {
   defines = [
     "NTDDI_VERSION=0x0A000000",
     "_WIN32_WINNT=0x0A00",
     "WINVER=0x0A00",
   ]
 }
 
 # Linker flags for Windows SDK setup, this is applied only to EXEs and DLLs.
 config("sdk_link") {
+  # Treat linker warnings as errors, to avoid spammy build output.
+  ldflags = [ "/WX" ]
+
   if (current_cpu == "x64") {
-    ldflags = [ "/MACHINE:X64" ]
+    ldflags += [ "/MACHINE:X64" ]
     lib_dirs = [
       "$windows_sdk_path\Lib\winv6.3\um\x64",
       "$visual_studio_path\VC\lib\amd64",
       "$visual_studio_path\VC\atlmfc\lib\amd64",
     ]
   } else {
-    ldflags = [
+    ldflags += [
       "/MACHINE:X86",
       "/SAFESEH",  # Not compatible with x64 so use only for x86.
       "/largeaddressaware",
     ]
     lib_dirs = [
       "$windows_sdk_path\Lib\winv6.3\um\x86",
       "$visual_studio_path\VC\lib",
       "$visual_studio_path\VC\atlmfc\lib",
     ]
   }
 }
 
 # This default linker setup is provided separately from the SDK setup so
 # targets who want different library configurations can remove this and specify
 # their own.
 config("common_linker_setup") {
   ldflags = [
+    "/WX",
     "/FIXED:NO",
     "/ignore:4199",
     "/ignore:4221",
     "/NXCOMPAT",
   ]
 
   ldflags += [

[brucedawson, 2016/12/21 22:22:53]

Odd that this is separate. For historical reasons I assume (since it is VC++
2015 only). Feel free to merge to clean this up. Or ignore.

[Wez, 2016/12/22 00:03:08]

Done.

     # Tell the linker to crash on failures.
     "/fastfail",
   ]
 
   # ASLR makes debugging with windbg difficult because Chrome.exe and
   # Chrome.dll share the same base name. As result, windbg will name the
   # Chrome.dll module like chrome_<base address>, where <base address>
   # typically changes with each launch. This in turn means that breakpoints in
   # Chrome.dll don't stick from one launch to the next. For this reason, we
   # turn ASLR off in debug builds.
@@ skipping 165 matching lines @@
 
 # Internal stuff --------------------------------------------------------------
 
 # Config used by the MIDL template to disable warnings.
 config("midl_warnings") {
   if (is_clang) {
     # MIDL generates code like "#endif !_MIDL_USE_GUIDDEF_".
     cflags = [ "-Wno-extra-tokens" ]
   }
 }