This makes it so that, given a |const nsIFrame*|, a caller can retrieve
properties but not set or remove them, but with an |nsIFrame*| all
operations are allowed. I believe this is sensible since properties act
as extended member variables for things that are needed rarely, and
these are the const-ness semantics of member variables.
This also avoids the need for const_cast<nsIFrame*> to cast away const
in the following patch, which guards property access with a frame state
bit.
MozReview-Commit-ID: IJ9JnGzdH51
e764ff99c9