Made some DOM fields private.

Relevant to #2242.
Improve the correctness of Root lifetimes

Use the new lifetime bounds feature of Rust to enforce that the
RootCollection field of Root outlives the jsref field. This still
doesn't enforce that the Root itself outlives the jsref field, and
doing this would require some sort of init dance.
Fix /python/servo code formatting.

(My editor screams at me for flake8 lint errors.)
Convert &JSRef -> JSRef for the PrivateNodeHelpers trait
Convert &JSRef -> JSRef in DisabledStateHelpers
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
Merge pull request #3493 from zwarich/root-lifetimes

Improve the correctness of Root lifetimes

Reviewed-by: jdm
Merge pull request #3496 from zwarich/disabled-state-helpers-jsref

Convert &JSRef -> JSRef in DisabledStateHelpers

Reviewed-by: jdm