[gn] Add back two warnings V8 uses with gyp

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 37d45d650a903ebb42518c6a7090c7a047bd1c93..ccecdb6deefa788bea4ddddaa5d8a331dc32053a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -371,6 +371,18 @@ config("toolchain") {
       "-fno-inline",
     ]
   }
+
+  if (is_clang) {
+    cflags += [
+      # TODO(hans): Remove once http://crbug.com/428099 is resolved.
+      "-Winconsistent-missing-override",

[Michael Achenbach, 2016/10/11 12:55:35]

I hope this actually switches it on. The gn defaults add
-Wno-inconsistent-missing-override and we append this afterwards.

[Clemens Hammacher, 2016/10/11 13:00:26]

A local test confirms that it should be switched on by this change.

[Nico, 2016/10/11 13:39:37]

This works by luck I think. The usual way is to put warning flags in a config
(https://cs.chromium.org/chromium/src/third_party/mesa/BUILD.gn?rcl=0&l=119)

[Michael Achenbach, 2016/10/11 13:54:31]

This is a config.

+    ]
+
+    if (v8_current_cpu == "x64" || v8_current_cpu == "arm64" ||
+        v8_current_cpu == "mips64el") {
+      cflags += [ "-Wshorten-64-to-32" ]
+    }
+  }
 }
 
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