Fix GC_PROFILING builds following r201671.

Source/platform/heap/ThreadState.cpp

Index: Source/platform/heap/ThreadState.cpp
diff --git a/Source/platform/heap/ThreadState.cpp b/Source/platform/heap/ThreadState.cpp
index 2864812a95a58e718f9de079e71b7f44214cf213..74f82581ba3b4195684f071eb3ccf48abed9c8cb 100644
--- a/Source/platform/heap/ThreadState.cpp
+++ b/Source/platform/heap/ThreadState.cpp
@@ -1495,7 +1495,7 @@ void ThreadState::takeSnapshot(SnapshotType type)
     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]);