Revert of Un-nest configs in GN files.

third_party/libpng/BUILD.gn

Index: third_party/libpng/BUILD.gn
diff --git a/third_party/libpng/BUILD.gn b/third_party/libpng/BUILD.gn
index 21420596db7fcaae359799e594aec3d1e320947f..c4e2650c8ae6ab2a1e51d4d2ee9f43cc1126e4f7 100644
--- a/third_party/libpng/BUILD.gn
+++ b/third_party/libpng/BUILD.gn
@@ -22,15 +22,6 @@
         "PNG_NO_MODULEDEF",
       ]
     }
-  }
-}
-
-# Must be in a config because of how GN orders flags (otherwise -Wall will
-# appear after this, and turn it back on).
-config("clang_warnings") {
-  if (is_clang) {
-    # Upstream uses self-assignment to avoid warnings.
-    cflags = [ "-Wno-self-assign" ]
   }
 }
 
@@ -71,6 +62,14 @@
     "//third_party/zlib",
   ]
 
+  # Must be in a config because of how GN orders flags (otherwise -Wall will
+  # appear after this, and turn it back on).
+  config("clang_warnings") {
+    if (is_clang) {
+      # Upstream uses self-assignment to avoid warnings.
+      cflags = [ "-Wno-self-assign" ]
+    }
+  }
   configs += [ ":clang_warnings" ]
 }