Summary:
This shows progress for the download of the JS bundle (different from the packager transform progress that we show already). This is useful especially when loading the JS bundle from a remote source or when developing on device (on simulator + localhost it pretty much just downloads instantly). This will be nice for the expo client since all bundles are loaded over the network and can take several seconds to load.
This depends on https://github.com/facebook/metro-bundler/pull/28 to work but won't crash or anything without it, it just won't show the progress percentage.
**Test plan**
Tested that bundle download progress is shown properly in RNTester on both localhost + simulator and on real device with network conditionner to simulate a slow loading bundle.
Tested that it doesn't cause issues if the packager doesn't send the Content-Length header.
Closes https://github.com/facebook/react-native/pull/15066
Differential Revision: D5449073
Pulled By: shergin
fbshipit-source-id: 43a8fb559393bbdc04f77916500e21898695bac5
ef23d2bdcf
