Chrome prototype: get browser-polyfill.js via build tools

[jaredhirsch]

Two tasks here:

1. Make sure outstanding issues with the browser-polyfill are resolved in that repo's master. Currently I believe feat: Added support for the sendResponse callback in the runtime.onMessage listeners mozilla/webextension-polyfill#97 is the one we need to land.

2. Add something to the build tools to fetch the browser-polyfill, build it, and add it to the Chrome build folder.

Refs #3998

[jaredhirsch]

Just heard from @rpl today that he's working with @Rob--W to finish up the PR. We can check on the PR in another week or two.

In the meantime, it would be good to update the polyfill to the latest version in the PR, as it might behave differently in some failure cases.

[jaredhirsch]

The upstream PR is still not merged. Probably need to kick this to next sprint.

[jaredhirsch]

The upstream PR has landed, but a new version hasn't been released yet. Filed mozilla/webextension-polyfill#150 to ask for a patch release.

[ianb]

I don't think we're going to launch on Chrome