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

Source/platform/PluginScriptForbiddenScope.cpp

Index: Source/platform/PluginScriptForbiddenScope.cpp
diff --git a/Source/platform/PluginScriptForbiddenScope.cpp b/Source/platform/PluginScriptForbiddenScope.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..64c1271ce25fa5ba68d81b863fd59f04a46e451d
--- /dev/null
+++ b/Source/platform/PluginScriptForbiddenScope.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/PluginScriptForbiddenScope.h"
+
+#include "wtf/Assertions.h"
+#include "wtf/MainThread.h"
+
+namespace blink {
+
+static unsigned s_pluginScriptForbiddenCount = 0;
+
+PluginScriptForbiddenScope::PluginScriptForbiddenScope()
+{
+    ASSERT(isMainThread());
+    ++s_pluginScriptForbiddenCount;
+}
+
+PluginScriptForbiddenScope::~PluginScriptForbiddenScope()
+{
+    ASSERT(isMainThread());
+    ASSERT(s_pluginScriptForbiddenCount);
+    --s_pluginScriptForbiddenCount;
+}
+
+bool PluginScriptForbiddenScope::isForbidden()
+{
+    ASSERT(isMainThread());
+    return s_pluginScriptForbiddenCount > 0;
+}
+
+} // namespace blink