修复不再使用的MutationObserver引起的内存泄漏问题

[LeoJiaXin]

Description

MutationObserver当前会被劫持，每个MutationObserver实例都会被缓存到observerSet中

改成：observe的时候缓存到observerSet，disconnect的时候删除实例

Related Issue

暂无

Motivation and Context

MutationObserver实例较多的时候，对于长时间使用的Web应用来说，内存会不断上涨，最终导致浏览器崩溃

How Has This Been Tested

打了js heap验证，在我们的项目目前是可以正常运行的

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

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