Clean up static stuff in dm.GeneratedContentNode

Remove enableAboutGrouping from GCNode and move it to the subclasses
so we can have GCNodes without grouping.

Having matchTagNames there doesn't make sense now that GCNode is a
mixin rather than a base class.

Also removed unused .static.generatedContent property.

Change-Id: I2b5350e70b7829cf009c752d066919e5a3593db0
Merge "Clean up static stuff in dm.GeneratedContentNode"