Bug 1343715 - Use SetDataFlags in ForgetSharedBuffer. r=dmajor

I believe this should fix some incorrect clearing of F_CLASS_FIXED.

MozReview-Commit-ID: 4ga2NEM9M5Z

--HG--
extra : transplant_source : %ECF%CF%D0%F6%19%9F%24%86%EFR%CAVZ%ED%60%D5nU%D8

xpcom/string/nsTSubstring.h

@@ -1045,7 +1045,7 @@ public:
     if (mFlags & nsAString::F_SHARED) {
       mData = char_traits::sEmptyBuffer;
       mLength = 0;
-      mFlags = F_TERMINATED;
+      SetDataFlags(F_TERMINATED);
     }
   }