Bug 1109928, part 1 - Move graph building inside CCGraphBuilder. r=smaug

This lets us hide a lot of the internal state of graph building from other classes.
Bug 1109928, part 2 - Inline some CCGraphBuilder methods now that the builder is doing the building. r=smaug

Also, make SetLastChild a private method defined in the class declaration.
Bug 1109928, part 3 - Make CCGraphBuilder::AddWeakMapNode and ::NoteJSChild methods private. r=smaug
Bug 1109928, part 4 - Make CCBuilder::AddNode private by adding a new AddPurpleRoot method. r=smaug
Bug 1109928, part 5 - Inline CCGraphBuilder::DescribeNode because it is silly. r=smaug
Bug 1109928, part 6 - Define CCGraphBuilder::SetFirstChild() for consistency. r=smaug