Turn on clang check for WeakPtrFactory member order

Note that class templates are currently ignored by all Chromium checkers; see crbug.com/441916.

BUG=303818

Review URL: https://codereview.chromium.org/803603003

Cr-Commit-Position: refs/heads/master@{#311770}

tools/clang/scripts/plugin_flags.sh

@@ -17,4 +17,5 @@ else
 fi
 
 echo -Xclang -load -Xclang $CLANG_LIB_PATH/libFindBadConstructs.$LIBSUFFIX \
-  -Xclang -add-plugin -Xclang find-bad-constructs
+  -Xclang -add-plugin -Xclang find-bad-constructs -Xclang \
+  -plugin-arg-find-bad-constructs -Xclang check-weak-ptr-factory-order