Linux/CrOS/Cast/Android (and Mac with gn): Build chromium_code with -Wextra.

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 97eb6726ca36455f5b87284c80ecc8c85f8c875e..ef3fc3400b48a018e690048adf85c7da43ffc485 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -894,15 +894,11 @@ config("chromium_code") {
   if (is_win) {
     cflags = [ "/W4" ]  # Warning level 4.
   } else {
-    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",
-    ]
+    cflags = [ "-Wall" ]
+    if (is_clang) {
+      # Enable -Wextra for chromium_code when we control the compiler.
+      cflags += [ "-Wextra" ]
+    }
 
     # In Chromium code, we define __STDC_foo_MACROS in order to get the
     # C99 macros on Mac and Linux.