win clang: don't disable -Wself-assign

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 787d4fa7d00ecb3b5e0dd237dd4cec24aa5d5a0e..056d91d61a9f0424b83bb7d954aff41c718af951 100644
--- a/third_party/libpng/BUILD.gn
+++ b/third_party/libpng/BUILD.gn
@@ -61,6 +61,16 @@ source_set("libpng_sources") {
   public_deps = [
     "//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" ]
 }
 
 if (is_win) {