linux: Build third-party code with -Wunused-result.

Part of an effort to make warning flags consistent across platforms.

BUG=574077

Review URL: https://codereview.chromium.org/1559903002

Cr-Original-Commit-Position: refs/heads/master@{#367355}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 09874b50f59f11ddd1268d6da87989ae09f07bfb

common.gypi

@@ -3158,15 +3158,6 @@
               '-Werror',
             ],
           }],
-          [ 'os_posix==1 and os_bsd!=1 and OS!="mac" and OS!="android"', {
-            'cflags': [
-              # Don't warn about ignoring the return value from e.g. close().
-              # This is off by default in some gccs but on by default in others.
-              # BSD systems do not support this option, since they are usually
-              # using gcc 4.2.1, which does not have this flag yet.
-              '-Wno-unused-result',
-            ],
-          }],
           [ 'OS=="win"', {
             'defines': [
               '_CRT_SECURE_NO_DEPRECATE',

config/compiler/BUILD.gn

@@ -946,14 +946,6 @@ config("no_chromium_code") {
     cflags += [ "-Wall" ]
   }
 
-  if (is_linux) {
-    # Don't warn about ignoring the return value from e.g. close(). This is
-    # off by default in some gccs but on by default in others. BSD systems do
-    # not support this option, since they are usually using gcc 4.2.1, which
-    # does not have this flag yet.
-    cflags += [ "-Wno-unused-result" ]
-  }
-
   if (is_clang) {
     cflags += [
       # Lots of third-party libraries have unused variables. Instead of