Oilpan: use a cross-thread persistent for per-isolate ScriptDebugServers.

Source/bindings/core/v8/V8PerIsolateData.cpp

Index: Source/bindings/core/v8/V8PerIsolateData.cpp
diff --git a/Source/bindings/core/v8/V8PerIsolateData.cpp b/Source/bindings/core/v8/V8PerIsolateData.cpp
index 476bb2c3da01efeddc8826309ecb4aaa06bebee9..96a7c99d7eeb44c6ddc759b969092988225a31f5 100644
--- a/Source/bindings/core/v8/V8PerIsolateData.cpp
+++ b/Source/bindings/core/v8/V8PerIsolateData.cpp
@@ -77,6 +77,7 @@ V8PerIsolateData::V8PerIsolateData()
     , m_internalScriptRecursionLevel(0)
 #endif
     , m_performingMicrotaskCheckpoint(false)
+    , m_debugServer(nullptr)
 {
     // FIXME: Remove once all v8::Isolate::GetCurrent() calls are gone.
     isolate()->Enter();
@@ -141,6 +142,7 @@ void V8PerIsolateData::destroy(v8::Isolate* isolate)
     V8PerIsolateData* data = from(isolate);
     // FIXME: Remove once all v8::Isolate::GetCurrent() calls are gone.
     isolate->Exit();
+    data->m_debugServer.clear();
     delete data;
 }