Handle DOM elements for service worker

[xiazhvera]

Issue #, if available:

Description of changes:

The polyfills is a workaround to introduce node APIs such as buffer and process in all javascript environments. In our original design, we set it up through window by window.Buffer = buffer.Buffer;.
However, window only exits in browser. In some of the javascript environments like service worker, the polypill would not work as window would not exit there.
Therefore, we change it to self. self gives us access to the global scope regardless of browser context. For example in the DOM, self would return window, while in a service worker self will return ServiceWorkerGlobalScope.
The fix should be able to bind the Buffer and process polyfills.

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