Reland of clang/win: Enable -Wunused-value completely. (patchset #1 id:1 of https://codereview.chromium.org/1501153002/ )

Reason for revert:
This might now work in release builds too.

Original issue's description:
> Revert of clang/win: Enable -Wunused-value completely. (patchset #3 id:40001 of https://codereview.chromium.org/1500783002/ )
>
> Reason for revert:
> This works in debug builds but not in release builds yet.
>
> Original issue's description:
> > clang/win: Enable -Wunused-value completely.
> >
> > BUG=505318
> > NOTRY=true
> >
> > Committed: https://crrev.com/b10a7ada601a07866d50cd13c6af4605e316152e
> > Cr-Commit-Position: refs/heads/master@{#363327}
>
> TBR=hans@chromium.org,wfh@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=505318
>
> Committed: https://crrev.com/9f1715d0cd8bb5c2bcfd00c55aff80e3fb0b7caf
> Cr-Commit-Position: refs/heads/master@{#363331}

TBR=hans@chromium.org,wfh@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=505318

Review URL: https://codereview.chromium.org/1507113002

Cr-Commit-Position: refs/heads/master@{#363618}

build/common.gypi

@@ -5813,10 +5813,6 @@
                   '-Wno-microsoft-enum-value',  # http://crbug.com/505296
                   '-Wno-unknown-pragmas',  # http://crbug.com/505314
                   '-Wno-microsoft-cast',  # http://crbug.com/550065
-                  # Disable unused-value (crbug.com/505318) except
-                  # -Wunused-result.
-                  '-Wno-unused-value',
-                  '-Wunused-result',
                 ],
               },
             }],

build/config/compiler/BUILD.gn

@@ -776,10 +776,6 @@ config("default_warnings") {
         "-Wno-microsoft-enum-value",  # http://crbug.com/505296
         "-Wno-unknown-pragmas",  # http://crbug.com/505314
         "-Wno-microsoft-cast",  # http://crbug.com/550065
-
-        # Disable unused-value (crbug.com/505318) except -Wunused-result.
-        "-Wno-unused-value",
-        "-Wunused-result",
       ]
     }
   } else {

chrome/installer/setup/setup_util_unittest.cc

@@ -617,7 +617,7 @@ TEST_F(DeleteRegistryKeyPartialTest, NonEmptyKeyWithPreserve) {
   {
     base::win::RegistryKeyIterator it(root_, path_.c_str());
     ASSERT_EQ(to_preserve_.size(), it.SubkeyCount());
-    for (it; it.Valid(); ++it) {
+    for (; it.Valid(); ++it) {
       ASSERT_NE(to_preserve_.end(),
                 std::find_if(to_preserve_.begin(), to_preserve_.end(),
                              [&it](const base::string16& key_name) {

sandbox/win/src/sid.cc

@@ -16,7 +16,7 @@ Sid::Sid(WELL_KNOWN_SID_TYPE type) {
   DWORD size_sid = SECURITY_MAX_SID_SIZE;
   BOOL result = ::CreateWellKnownSid(type, NULL, sid_, &size_sid);
   DCHECK(result);
-  DBG_UNREFERENCED_LOCAL_VARIABLE(result);
+  (void)result;
 }
 
 const SID *Sid::GetPSID() const {