Fix GC_PROFILING builds following r201671.

Source/platform/heap/ThreadState.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 1477 matching lines @@
     if (type == SnapshotType::FreelistSnapshot)
         return;
 
     size_t totalLiveCount = 0;
     size_t totalDeadCount = 0;
     size_t totalLiveSize = 0;
     size_t totalDeadSize = 0;
     for (size_t gcInfoIndex = 1; gcInfoIndex <= GCInfoTable::gcInfoIndex(); ++gcInfoIndex) {
         String dumpName = classesDumpName + String::format("/%lu_", static_cast<unsigned long>(gcInfoIndex));
 #if ENABLE(GC_PROFILING)
-        dumpName += Heap::gcInfo(gcInfoIndex)->className();
+        dumpName.append(Heap::gcInfo(gcInfoIndex)->className());
 #endif
         WebMemoryAllocatorDump* classDump = BlinkGCMemoryDumpProvider::instance()->createMemoryAllocatorDumpForCurrentGC(dumpName);
         classDump->AddScalar("live_count", "objects", info.liveCount[gcInfoIndex]);
         classDump->AddScalar("dead_count", "objects", info.deadCount[gcInfoIndex]);
         classDump->AddScalar("live_size", "bytes", info.liveSize[gcInfoIndex]);
         classDump->AddScalar("dead_size", "bytes", info.deadSize[gcInfoIndex]);
 
         totalLiveCount += info.liveCount[gcInfoIndex];
         totalDeadCount += info.deadCount[gcInfoIndex];
         totalLiveSize += info.liveSize[gcInfoIndex];
@@ skipping 101 matching lines @@
         json->beginArray(it->key.ascii().data());
         for (size_t age = 0; age <= maxHeapObjectAge; ++age)
             json->pushInteger(it->value.ages[age]);
         json->endArray();
     }
     TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(TRACE_DISABLED_BY_DEFAULT("blink_gc"), statsName, this, json.release());
 }
 #endif
 
 } // namespace blink