web_dev_style: ignore --css-vars from alphabetical ordering requirement

chrome/browser/web_dev_style/css_checker_test.py

Index: chrome/browser/web_dev_style/css_checker_test.py
diff --git a/chrome/browser/web_dev_style/css_checker_test.py b/chrome/browser/web_dev_style/css_checker_test.py
index 1866614ab61ee9e25d523e79e8c58f6bc85303c0..0883ff6e86f7026c0c837444f11b05c5faf0e4d1 100755
--- a/chrome/browser/web_dev_style/css_checker_test.py
+++ b/chrome/browser/web_dev_style/css_checker_test.py
@@ -133,6 +133,13 @@ div {
     border-left: 5px;
     border: 5px solid red;""")
 
+  def testAlphaIgnoresVariables(self):
+    self.VerifyContentsIsValid("""
+#id {
+  --zzyxx-xylophone: 3px;
+  --aardvark-animal: var(--zzyxz-xylophone);

[dschuyler, 2016/09/15 23:17:36]

maybe add:

--two-line:
    var(--second-line);

--like-a-mixin: {
  foo: 4px;
  bar: 5px;
};

[dschuyler, 2016/09/15 23:48:35]

With my better understanding of the mixins being
removed first, I see that the --like-a-mixin test
is not a good addition.

+}""")
+
   def testCssBracesHaveSpaceBeforeAndNothingAfter(self):
     self.VerifyContentsProducesOutput("""
 /* Hello! */div/* Comment here*/{