Presubmit test for overriding properties generated by Polymer mixin shim

chrome/browser/web_dev_style/css_checker.py

Index: chrome/browser/web_dev_style/css_checker.py
diff --git a/chrome/browser/web_dev_style/css_checker.py b/chrome/browser/web_dev_style/css_checker.py
index 93897e92dd8427f96acf5f7266e43132c0e06beb..77f610f4db804a7f0c0e52f8400ad54333d9de5d 100644
--- a/chrome/browser/web_dev_style/css_checker.py
+++ b/chrome/browser/web_dev_style/css_checker.py
@@ -169,6 +169,9 @@ class CSSChecker(object):
     def no_data_uris_in_source_files(line):
       return re.search(r'\(\s*\s*data:', line)
 
+    def no_mixin_shims(line):
+      return re.search('\-\-[\w\-]+_\-_[\w\-]+\s*:', line)
+
     def no_quotes_in_url(line):
       return re.search('url\s*\(\s*["\']', line, re.IGNORECASE)
 
@@ -346,6 +349,10 @@ class CSSChecker(object):
         { 'desc': "Don't use data URIs in source files. Use grit instead.",
           'test': no_data_uris_in_source_files,
         },
+        { 'desc': "Don't override custom properties created by Polymer's mixin "
+                  "shim. Set mixins or documented custom properties directly.",
+          'test': no_mixin_shims,
+        },
         { 'desc': "Don't use quotes in url().",
           'test': no_quotes_in_url,
         },