Fix presubmit @2x recognized as @at-keyword.

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 d626e6098d5cfd7529c70f036ab2a184b959fcbd..494888b0d5a78cb09800fe1b68152647eaea854a 100644
--- a/chrome/browser/web_dev_style/css_checker.py
+++ b/chrome/browser/web_dev_style/css_checker.py
@@ -33,9 +33,9 @@ class CSSChecker(object):
 
     def _remove_ats(s):
       at_reg = re.compile(r"""
-          @\w+[^'"]*?{  # @at-keyword selector junk {
-          (.*{.*?})+    # inner { curly } blocks, rules, and selector junk
-          .*?}          # stuff up to the first end curly }""",
+          @(?!\d+x\b)\w+[^'"]*?{  # @at-keyword selector junk {, not @2x
+          (.*{.*?})+              # inner { curly } blocks, rules, and selector
+          .*?}                    # stuff up to the first end curly }""",
           re.DOTALL | re.VERBOSE)
       return at_reg.sub('\\1', s)