clang/win: Let some chromium_code targets build 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 49b3737f65188133134922267d7ac9da41d1c5f8..6a3d154035afe314a92f6c43a5b80e29acf33f36 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -783,16 +783,6 @@ config("default_warnings") {
       ]
     }
   } else {
-    # Common GCC warning setup.
-    cflags += [
-      # Enables.
-      "-Wendif-labels",  # Weird old-style text after an #endif.
-
-      # Disables.
-      "-Wno-missing-field-initializers",  # "struct foo f = {0};"
-      "-Wno-unused-parameter",  # Unused function parameters.
-    ]
-
     if (treat_warnings_as_errors) {
       cflags += [ "-Werror" ]
     }
@@ -834,6 +824,18 @@ config("default_warnings") {
     }
   }
 
+  # Common Clang and GCC warning setup.
+  if (!is_win || is_clang) {
+    cflags += [
+      # Enables.
+      "-Wendif-labels",  # Weird old-style text after an #endif.
+
+      # Disables.
+      "-Wno-missing-field-initializers",  # "struct foo f = {0};"
+      "-Wno-unused-parameter",  # Unused function parameters.
+    ]
+  }
+
   if (is_clang) {
     cflags += [
       # This warns on using ints as initializers for floats in