Issue #2089 - Implement AggregateError

Based-on: m-c 1568903/4,1641355(partial),1652148,1643397

js/xpconnect/src/XPCJSContext.cpp

@@ -108,6 +108,7 @@ const char* const XPCJSContext::mStrings[] = {
     "columnNumber",         // IDX_COLUMNNUMBER
     "stack",                // IDX_STACK
     "message",              // IDX_MESSAGE
+    "errors",               // IDX_ERRORS
     "lastIndex"             // IDX_LASTINDEX
 };
 

js/xpconnect/src/xpcprivate.h

@@ -495,6 +495,7 @@ public:
         IDX_COLUMNNUMBER            ,
         IDX_STACK                   ,
         IDX_MESSAGE                 ,
+        IDX_ERRORS                  ,
         IDX_LASTINDEX               ,
         IDX_TOTAL_COUNT // just a count of the above
     };