This is the same optimization made for ThreadSafeAutoRefCnt in bug
1277709. However, nsStringBuffer uses a 32-bit reference count all the
time, so it can't easily use ThreadSafeAutoRefCnt.
MozReview-Commit-ID: LpB3xaYbaEE
The patch changes all uses of SizeOfIncludingThisMustBeUnshared() to
SizeOfIncludingThisIfUnshared(). This incurs the (tiny) cost of an unnecessary
IsReadonly() check for guaranteed-unshared strings, but avoids the possible
assertion failures that would occur when MustBeUnshared() was used incorrectly
on shared strings, which is an easy mistake to make.
