There are concerns about performance impacts on setting GridColTrackSizes/GridRowTrackSizes on every reflow. We should only set those frame properties when they are needed.

We will have the same problem with the data we want to provide for the DevTools API (bug 1241932). Perhaps we can solve it by making the consumers of the data set a bit on the GridContainerFrame, call FrameNeedsReflow on it, then Flush_Layout. GridContainerFrame::Reflow can then setup the frame properties only if requested by those bits. Then we only pay the price when the data is actually requested, at the cost of an extra (likely unnecessary) reflow on the first request.