Make the metamask-extension-provider work with privacy mode

The steps described [in this blogpost](https://medium.com/metamask/https-medium-com-metamask-breaking-change-injecting-web3-7722797916a8) do not work when connecting from another browser extension (i.e. when using the [metamask-extension-provider](https://github.com/MetaMask/metamask-extension-provider)).

[We](https://github.com/SuperuserLabs/thankful) really hope this will be fixed before privacy mode is turned opt-out, since our app would otherwise break for most users.

Read the linked privacy-mode blog post and the metamask-extension-provider repository first, then reproduce the connection from another browser extension. Trace how privacy mode changes extension-to-extension access. Done means the provider works with privacy mode enabled without breaking the reported app integration.

Written by the indexing model from the issue text.