Fix tautological comparison and add clang flag

Review-Url: https://codereview.chromium.org/2581733002
Cr-Original-Commit-Position: refs/heads/master@{#439346}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1b142eb67aedcd40f07234e38174b5b0f9995dc5

config/clang/BUILD.gn

@@ -56,5 +56,7 @@ config("extra_warnings") {
 
     # Warns when a const char[] is converted to bool.
     "-Wstring-conversion",
+
+    "-Wtautological-overlap-compare",
   ]
 }