Clang style plugin: add warn-only option and use it on Windows

build/config/clang/BUILD.gn

Index: build/config/clang/BUILD.gn
diff --git a/build/config/clang/BUILD.gn b/build/config/clang/BUILD.gn
index 3d965003b89ac0632f7d0fb63213fde81aa14f37..e79a7b93d82eb4ea9e74449cc46bb64426f8a792 100644
--- a/build/config/clang/BUILD.gn
+++ b/build/config/clang/BUILD.gn
@@ -31,6 +31,16 @@ config("find_bad_constructs") {
       ]
     }
 
+    if (is_win) {
+      # Don't error on plugin warnings on Windows until pre-existing warnings
+      # are cleaned up.  https://crbug.com/467287
+      cflags += [
+        "-Xclang",
+        "-plugin-arg-find-bad-constructs",
+        "-Xclang warn-only",
+      ]
+    }
+
     cflags += [
       "-Xclang",
       "-add-plugin",