Add fatal_linker_warnings flag for GN builds

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 78c26e391bb6f409c8027c168c1b6dab9f5b045f..50ee2a6a40da282b425d210b0a17a74618e64d5e 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -81,6 +81,11 @@ declare_args() {
   # conjunction with setting LLVM_FORCE_HEAD_REVISION in the
   # environment when `gclient runhooks` is run as well.
   llvm_force_head_revision = false
+
+  # Enable fatal linker warnings. Building Chromium with certain versions
+  # of binutils can cause linker warning.
+  # See: https://bugs.chromium.org/p/chromium/issues/detail?id=457359
+  fatal_linker_warnings = true

[Dirk Pranke, 2016/06/30 20:00:41]

We don't support building Chromium on 32-bit linux hosts any more (you need a
64-bit system to build, even if you're going to run on 32-bit targets).

So, linking to that bug isn't a good example. Are there other bugs that we can
link against? Maybe you can point to a current need for this flag?

 }
 
 if (is_clang && !is_nacl) {
@@ -203,7 +208,8 @@ config("compiler") {
     }
 
     # Linker warnings.
-    if (!(is_chromeos && current_cpu == "arm") &&
+    if ( fatal_linker_warnings &&
+        !(is_chromeos && current_cpu == "arm") &&
         !(is_android && use_order_profiling) && !is_mac && !is_ios) {
       # TODO(jochen): Enable this on chromeos on arm. http://crbug.com/356580
       # TODO(lizeb,pasko): Fix link errors when linking with order_profiling=1