Enforce WARN_UNUSED_RESULT attribute on Windows

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index cce7b3ca56ed001a1e3f08c54cc1e076e68aee53..7ea18a8aa8092ae93d1d23889a5d66be06d5e4dc 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -5764,6 +5764,7 @@
                   '-Wno-microsoft-enum-value',  # http://crbug.com/505296
                   '-Wno-unknown-pragmas',  # http://crbug.com/505314
                   '-Wno-unused-value',  # http://crbug.com/505318
+                  '-Wunused-result',

[dcheng, 2015/10/07 16:09:59]

/W4 doesn't seem to enable this warning, so I had to manually add it to this
list.

[hans, 2015/10/07 16:25:26]

Hmm, clang-cl /W4 does enable it for me. Can you double check? That we're
disabling -Wunused-value above suggests that these warnings are on by default.

Also, this is a list of disabled warnings (see the comment about making it
shorter above). If we need to enable this explicitly, let's move it to
clang_warning_flags above.

[dcheng, 2015/10/07 16:41:34]

Hm...

Well it's definitely not enabled by default on the Chrome build, but I have no
idea what flag is causing it yet.

(It's easy enough to prove this by patching in just the compiler_specific.h
changes and then introducing an intentional error in base_unittests somewhere)

[hans, 2015/10/07 16:51:29]

D'oh, it's right there: -Wno-unused-value also disables the unused-result
warning. Sorry, I should have noticed that.

Maybe something like:

  # Disable unused-value (crbug.com/505318) except -Wunused-result.
  '-Wno-unused-value',
  '-Wunused-result',

[dcheng, 2015/10/07 17:46:15]

Done.

                 ],
               },
             }],