Enforce WARN_UNUSED_RESULT attribute on Windows

BUG=none
R=cpu,danakj,dpranke,hans,kelvinp,rickyz,sky,sorin,thestig,tuttle,yukawa
TBR=vitalybuka

Review URL: https://codereview.chromium.org/1390223002

Cr-Original-Commit-Position: refs/heads/master@{#353450}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7764fe9ea52233096b7916614eb7c60b26bbca05

common.gypi

@@ -5780,7 +5780,10 @@
                   '-Qunused-arguments',  # http://crbug.com/504658
                   '-Wno-microsoft-enum-value',  # http://crbug.com/505296
                   '-Wno-unknown-pragmas',  # http://crbug.com/505314
-                  '-Wno-unused-value',  # http://crbug.com/505318
+                  # Disable unused-value (crbug.com/505318) except
+                  # -Wunused-result.
+                  '-Wno-unused-value',
+                  '-Wunused-result',
                 ],
               },
             }],

config/compiler/BUILD.gn

@@ -637,7 +637,10 @@ config("default_warnings") {
         "-Qunused-arguments",  # http://crbug.com/504658
         "-Wno-microsoft-enum-value",  # http://crbug.com/505296
         "-Wno-unknown-pragmas",  # http://crbug.com/505314
-        "-Wno-unused-value",  # http://crbug.com/505318
+
+        # Disable unused-value (crbug.com/505318) except -Wunused-result.
+        "-Wno-unused-value",
+        "-Wunused-result",
       ]
     }
   } else {