Bug 1390036 (part 1) - Remove most remaining uses of nsXPIDLString. r=erahm.

CompareCacheHashEntry::mCrit[] is the only case where the nsXPIDLString-ness
was important. The patch adds an explicit SetIsVoid() call to that class's
constructor and changes some null checks to IsVoid() checks.

docshell/base/nsDocShell.cpp

@@ -9730,7 +9730,6 @@ public:
 
 private:
   // Use IDL strings so .get() returns null by default
-  nsXPIDLString mWindowTarget;
   nsXPIDLCString mTypeHint;
   nsString mSrcdoc;