Add a scoping object to help block scripting during plugin destruction.

Source/platform/ReentrantScriptForbiddenScope.cpp

Index: Source/platform/ReentrantScriptForbiddenScope.cpp
diff --git a/Source/platform/ReentrantScriptForbiddenScope.cpp b/Source/platform/ReentrantScriptForbiddenScope.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..c2a4c135e9d3c792745b7adcfb955dcbdb4895eb
--- /dev/null
+++ b/Source/platform/ReentrantScriptForbiddenScope.cpp
@@ -0,0 +1,34 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "config.h"
+#include "platform/ReentrantScriptForbiddenScope.h"
+
+#include "wtf/Assertions.h"
+#include "wtf/MainThread.h"
+
+namespace blink {
+
+static unsigned s_reentrantScriptForbiddenCount = 0;
+
+ReentrantScriptForbiddenScope::ReentrantScriptForbiddenScope()
+{
+    ASSERT(isMainThread());
+    ++s_reentrantScriptForbiddenCount;
+}
+
+ReentrantScriptForbiddenScope::~ReentrantScriptForbiddenScope()
+{
+    ASSERT(isMainThread());
+    ASSERT(s_reentrantScriptForbiddenCount);
+    --s_reentrantScriptForbiddenCount;
+}
+
+bool ReentrantScriptForbiddenScope::isForbidden()
+{
+    ASSERT(isMainThread());
+    return s_reentrantScriptForbiddenCount > 0;
+}
+
+} // namespace blink