Make it possible to disable fatal linker warnings

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index cff6f608d5f00c0e533b23d92c2f05b5ce3b68a6..eb0d86f7039216646b4478c14c7b15c7342fe338 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -1230,6 +1230,10 @@
     # whether warnings are treated as errors.
     'chromium_code%': 0,
 
+    # Disable fatal linker warnings, similarly to how we make it possible
+    # to disable -Werror (e.g. for different toolchain versions).
+    'disable_fatal_linker_warnings%': 0,
+
     'release_valgrind_build%': 0,
 
     # TODO(thakis): Make this a blacklist instead, http://crbug.com/101600
@@ -3069,16 +3073,22 @@
     },
   },
   'conditions': [
-    # TODO(jochen): Enable this on chromeos on arm. http://crbug.com/356580
-    ['os_posix==1 and (chromeos==0 or target_arch!="arm")', {
+    ['os_posix==1', {
       'target_defaults': {
         'ldflags': [
-          '-Wl,--fatal-warnings',
           '-Wl,-z,now',
           '-Wl,-z,relro',
         ],
       },
     }],
+    # TODO(jochen): Enable this on chromeos on arm. http://crbug.com/356580
+    ['os_posix==1 and disable_fatal_linker_warnings==0 and (chromeos==0 or target_arch!="arm")', {
+      'target_defaults': {
+        'ldflags': [
+          '-Wl,--fatal-warnings',
+        ],
+      },
+    }],
     ['os_posix==1 and chromeos==0', {
       # Chrome OS enables -fstack-protector-strong via its build wrapper,
       # and we want to avoid overriding this, so stack-protector is only