[gn] Add back two warnings V8 uses with gyp

BUG=428099

Review-Url: https://codereview.chromium.org/2404283002
Cr-Commit-Position: refs/heads/master@{#40235}

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