Add exceptions to features in GpuControlList

gpu/config/gpu_control_list_format.txt

Index: gpu/config/gpu_control_list_format.txt
diff --git a/gpu/config/gpu_control_list_format.txt b/gpu/config/gpu_control_list_format.txt
index 4e2df4275ae94b8434d909ca89522885d5faccbb..7b23ad86dff7c9ab035e1a583939e13137e14e16 100644
--- a/gpu/config/gpu_control_list_format.txt
+++ b/gpu/config/gpu_control_list_format.txt
@@ -52,6 +52,8 @@
 // 20. "features" is a list of gpu control list options, which can be
 //     configured by a specific list. See its *_json.cc file for a list of
 //     supported features. This field is mandatory.
+//     An "exceptions" list to the features can be added for convenience instead
+//     of listing all features except one.
 // 21. "description" has the description of the entry.
 // 22. "webkit_bugs" is an array of associated webkit bug numbers.
 // 23. "cr_bugs" is an array of associated webkit bug numbers.