Enable Wextra and Wno-unused-parameters in default cflags

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 5bbf6195df155d1917fbfd9700ec758168992fbe..cc2b7ac3325d2698a1757a22342bfcf75c118ef8 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -823,6 +823,8 @@ if (is_clang) {
     # code generated by flex (used in angle) contains that keyword.
     # http://crbug.com/255186
     "-Wno-deprecated-register",
+
+    "-Wno-unused-parameter",
   ]
 
   # NaCl's Clang compiler and Chrome's hermetic Clang compiler will almost
@@ -845,12 +847,7 @@ if (is_clang) {
 config("chromium_code") {
   cflags = [
     "-Wall",
-
-    # GCC turns on -Wsign-compare for C++ under -Wall, but clang doesn't,
-    # so we specify it explicitly.
-    # TODO(fischman): remove this if http://llvm.org/PR10448 obsoletes it.
-    # http://code.google.com/p/chromium/issues/detail?id=90453
-    "-Wsign-compare",
+    "-Wextra",
   ]
 
   # In Chromium code, we define __STDC_foo_MACROS in order to get the