Fix all "dangerous-default-value" pylint warnings.

third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem.py

Index: third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem.py
diff --git a/third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem.py b/third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem.py
index 63a84c786a7bd8ec7d1dd7c64bc9a2a0af6d3ffa..461d27d9da7d9b6efb8744352177ad6586ab5371 100644
--- a/third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem.py
+++ b/third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem.py
@@ -79,7 +79,7 @@ class FileSystem(object):
     def exists(self, path):
         return os.path.exists(path)
 
-    def files_under(self, path, dirs_to_skip=[], file_filter=None):
+    def files_under(self, path, dirs_to_skip=None, file_filter=None):
         """Return the list of all files under the given path in topdown order.
 
         Args:
@@ -90,6 +90,8 @@ class FileSystem(object):
                 each file found. The file is included in the result if the
                 callback returns True.
         """
+        dirs_to_skip = dirs_to_skip or []
+
         def filter_all(fs, dirpath, basename):
             return True