Bug 847518 - window.open(url, "_blank") should open in default browser. r=myk, r=bz
Bug 952051 - Change the GetOperator call in FlushGroup to take into account the mix-blend-mode operator for the group. r=roc

Create a new GetEffectiveOperator function that also takes into acount the mix-blend-mode operator for basic layers. Replace logic that chooses between the effective mix blend mode and the operator returned by GetOperator, with calls to this function.
Bug 952051 - Add reftests. r=roc

* Add a reftest for checking that blending takes place between a blended child element that is clipped by a rounded rect and its parent element (which has overflow hidden, and rounded corners).
* Add a reftest to check the behavior described in bug 947121 no longer reproduces.