Fix assertion failure in ScriptRegexp::match

Source/platform/ScriptForbiddenScope.cpp

 // Copyright 2014 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/ScriptForbiddenScope.h"
 
 #include "wtf/Assertions.h"
 #include "wtf/MainThread.h"
 
@@ skipping 26 matching lines @@
     ASSERT(s_scriptForbiddenCount);
     --s_scriptForbiddenCount;
 }
 
 bool ScriptForbiddenScope::isScriptForbidden()
 {
     return isMainThread() && s_scriptForbiddenCount;
 }
 
 ScriptForbiddenScope::AllowUserAgentScript::AllowUserAgentScript()
-    : m_change(s_scriptForbiddenCount, 0)
 {
-    ASSERT(isMainThread());
+    if (isMainThread())
+        m_change.emplace(s_scriptForbiddenCount, 0);
 }
 
 ScriptForbiddenScope::AllowUserAgentScript::~AllowUserAgentScript()
 {
-    ASSERT(!s_scriptForbiddenCount);
+    ASSERT(!isMainThread() || !s_scriptForbiddenCount);
 }
 
 } // namespace blink