RenderBlock::layoutPositionedObjects should use setNeedsPositionedMovementLayout

This CL swiches RenderBlock::layoutPositionedObjects from using setNeedsLayout
to setNeedsPositionedMovementLayout, as indicated by the FIXME comment and
associated bug. The test failure mentioned in the bug is no longer a problem.

BUG=350756

Review URL: https://codereview.chromium.org/414123006

git-svn-id: svn://svn.chromium.org/blink/trunk@179289 bbb929c8-8fbe-4397-9dbb-9b2b20218538