Bug 1055285 part 1: Implement CSS parsing & computation for 'object-fit' property. r=heycam
Bug 1055285 part 2: Implement CSS parsing & computation (but not animation) for 'object-position' property. r=heycam
Bug 1055285 part 3: Implement animation for 'object-position' property. r=heycam
Bug 1055285 part 4: Extend test_transitions_per_property.html with a few more scenarios for background-position & object-position transitions. (no review)