Bug 1373672 - Part 2: nsDocShell::mChildOffset should be signed, r=smaug

All consumers of this value expect the passed-in value to be signed, and a
negative value is stored into this variable (-1) when the docshell was
dynamically added. It makes more sense for this to be signed.

MozReview-Commit-ID: 8iKDOAx7O2R

docshell/base/nsDocShell.cpp

@@ -4083,7 +4083,7 @@ nsDocShell::SetTreeOwner(nsIDocShellTreeOwner* aTreeOwner)
 }
 
 NS_IMETHODIMP
-nsDocShell::SetChildOffset(uint32_t aChildOffset)
+nsDocShell::SetChildOffset(int32_t aChildOffset)
 {
   mChildOffset = aChildOffset;
   return NS_OK;