Merge pull request #448 from mithro/sauce-pull-fix-2

Second attempt at fixing the secure access problem.
Teach Web Animations about step-start, step-middle, step-end shorthands.
returnValue is part of BeforeUnloadEvent
Merge pull request #451 from arv/remove-return-value

returnValue should not be part of TimingEvent
Merge pull request #449 from shans/step-shorthands

Teach Web Animations about step-start, step-middle, step-end shorthands.
Merge remote-tracking branch 'up/master' into enable-linter