Made some DOM fields private.

Relevant to #2242.
Fix /python/servo code formatting.

(My editor screams at me for flake8 lint errors.)
Convert &JSRef -> JSRef for the PrivateNodeHelpers trait
Merge pull request #3494 from SimonSapin/flake8

Fix /python/servo code formatting.

Reviewed-by: larsbergstrom
Handle dynamic addition of attributes

Handles dynamic addition of attributes to <link rel=stylesheet>
elements.

Fixes #3361
Merge pull request #3495 from zwarich/private-node-helpers-jsref

Convert &JSRef -> JSRef for the PrivateNodeHelpers trait

Reviewed-by: jdm
Merge pull request #3432 from gilles-leblanc/issue-3361

Handle dynamic addition of attributes

Reviewed-by: jdm
Merge pull request #3489 from ProgramFOX/issue2242-2

Made some DOM fields private.

Reviewed-by: jdm