This patch adds three test cases;
1) Animation on position:absolute element in a zero-height iframe
This animation should be throttled.
2) Animation on a non-zero width and hight position:absolute element but whose
parent has a zero height
This animation should NOT be throttled since the animation is visible
3) Animation on a zero-height position:absolute element whose parent also has
zero height.
This animation should be throttled since the animation is invisible
The first test fails without this fix and passes with the fix.
The second one passes regardless of the fix
The third one is marked as 'todo' since it doesn't pass with this fix.
MozReview-Commit-ID: 8pNUFQ71ivj
b7f506f202