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

build/config/win/BUILD.gn

Index: build/config/win/BUILD.gn
diff --git a/build/config/win/BUILD.gn b/build/config/win/BUILD.gn
index 3da27470120c5fe17c564f6f012dff4694cbc627..8885a70bf363b075056af2f88661769e5e069da7 100644
--- a/build/config/win/BUILD.gn
+++ b/build/config/win/BUILD.gn
@@ -102,7 +102,7 @@ config("compiler") {
         # 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",
@@ -212,15 +212,18 @@ config("winver") {
 
 # 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",
@@ -238,6 +241,7 @@ config("sdk_link") {
 # their own.
 config("common_linker_setup") {
   ldflags = [
+    "/WX",
     "/FIXED:NO",
     "/ignore:4199",
     "/ignore:4221",