Only not take zOffset into account during hit-testing when child layers are in the same 3d rendering context
Currently, hit-testing computes zOffset of layers with transformations. When
two layers overlap, it returns the layer with the highest zOffset.
Hit-testing should behave this way only when preserve3d is true.
BUG=
126479,
310464
Committed:
https://src.chromium.org/viewvc/blink?view=rev&revision=177685