Fix MockFileSystem.walk and add test.

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

Index: third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem_mock.py
diff --git a/third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem_mock.py b/third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem_mock.py
index 6b6e7c9f422f91384cdd33c5fac50ab0387471e3..81ea3ac03c17957c129f3ef0c38fc631264d30e4 100644
--- a/third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem_mock.py
+++ b/third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem_mock.py
@@ -205,6 +205,7 @@ class MockFileSystem(object):
         return dirs + files
 
     def walk(self, top):
+        FILE_SYSTEM_TUPLES = []

[qyearsley, 2016/06/01 23:08:43]

Non-global variable names use lowercase with underscore, e.g.
file_system_tuples.

         sep = self.sep
         if not self.isdir(top):
             raise OSError("%s is not a directory" % top)
@@ -223,7 +224,12 @@ class MockFileSystem(object):
                         dirs.append(dir)
                 else:
                     files.append(remaining)
-        return [(top[:-1], dirs, files)]
+        FILE_SYSTEM_TUPLES = [(top[:-1], dirs, files)]
+        for dir in dirs:
+            dir = top + dir
+            subdir = self.walk(dir)

[qyearsley, 2016/06/01 23:08:43]

This variable name could probably be improved; is it a tuple or a list of
tuples? The name `subdir` suggests that it may be a string.

+            FILE_SYSTEM_TUPLES += subdir
+        return FILE_SYSTEM_TUPLES
 
     def mtime(self, path):
         if self.exists(path):