Container status should be updated when adding or removing transforms.

Updating the positioned descendant list of a container requires a layout.
Previously we request a simplified normal flow layout for all transform
changes, and the unneeded layout was eliminated in a recent CL.

This CL changes RenderBlock::styleWillChange(), which originally only looks
at the position property to determine whether the positioned descendant list
needs to be updated. Now it also looks at transforms.

BUG=373780
R=ojan

Review URL: https://codereview.chromium.org/291273003

git-svn-id: svn://svn.chromium.org/blink/trunk@174481 bbb929c8-8fbe-4397-9dbb-9b2b20218538