Bug 1229458 - Remove SizeOfIncludingThisMustBeUnshared() from string classes. r=mccr8.

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.

xpcom/components/nsComponentManager.cpp

@@ -1846,7 +1846,7 @@ nsComponentManagerImpl::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf)
   for (auto iter = mContractIDs.ConstIter(); !iter.Done(); iter.Next()) {
     // We don't measure the nsFactoryEntry data because it's owned by
     // mFactories (which is measured above).
-    n += iter.Key().SizeOfExcludingThisMustBeUnshared(aMallocSizeOf);
+    n += iter.Key().SizeOfExcludingThisIfUnshared(aMallocSizeOf);
   }
 
   n += sStaticModules->ShallowSizeOfIncludingThis(aMallocSizeOf);