Bug 917021:  We should not store the NormalPositionProperty on all frames, only relatively positioned ones.  r=dholbert  approval-mozilla-aurora=lsblakk

This also fixes nsHTMLReflowState::ApplyRelativePositioning to no-op for
svg:text, as it should (since nsIFrame::IsRelativelyPositioned() checks
more than nsStyleDisplay::IsRelativelyPositionedStyle()).
Bug 916751:  Do not use nsIFrame::MovePositionBy from nsLineLayout.  r=dholbert  approval-mozilla-aurora=lsblakk

Since bug 916751 is hard (for me) to test, I haven't confirmed for sure
that this fixes the bug.

However, it fixes the assertions that bug 911786 part 3 triggers in
layout/base/crashtests/317934-1.html through this codepath.
Bug 911786 part 3:  Add an assertion to check that part 2 only changes things for position:sticky.  r=dholbert  approval-mozilla-aurora=debug-only

I'm worried that part 2 was the cause of bug 916751.  If it was, I think
this assertion should catch the problem.  Even if it's not, I think it's
a good assertion to have.