mac and ios: Turn linker warnings into errors.

Bug: 747727
Change-Id: I33f44e3f4011873e193088796483ecdc48f7f05e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720610Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681434}

build/config/compiler/BUILD.gn

@@ -82,7 +82,6 @@ declare_args() {
 
   # 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
 
   # Build with C++ RTTI enabled. Chromium builds without RTTI by default,
@@ -301,6 +300,9 @@ config("compiler") {
       # crbug.com/485542
       ldflags += [ "-Wl,--fatal-warnings" ]
     }
+    if (fatal_linker_warnings && (is_mac || is_ios)) {
+      ldflags += [ "-Wl,-fatal_warnings" ]
+    }
   } else {
   }