Add electron enviroment check to http headers overwrite

[aprTaylor]

Checks for Electron environment before overwriting HTTPS headers.

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[aprTaylor]

@googlebot I signed it!

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[aprTaylor]

@googlebot I fixed it.

[aprTaylor]

@googlebot I fixed it.

[Feiyang1]

Hi @agray5 Can you please tell us what the motivation is for this change?
We added this check to workaround the header issues in react native. #703

Are you facing a similar issue in Electron?

[mikelehen]

@Feiyang1 FYI- The context here is #1491. It turns out this issue happens whenever the browser environment doesn't send an Origin header. React Native ran into this, but Electron/7 and other environments seem to also be hitting it.

[mikelehen]

navigator may not be defined in some environments (e.g. node.js) and so this could crash. The easy solution is to use the existing getUA() helper.

[mikelehen]

Thanks @agray5 I ended up just rolling this up into https://github.com/firebase/firebase-js-sdk/pull/2464/files which I will try to get checked in and released sometime in January. Appreciate the help!