Add platform hook for View style props

[rozele]

Similar to the ViewPropTypes hook to allow customization of prop types for iOS and Android, it's also useful to allow platforms to override style prop types.

Motivation

(Write your motivation here.)

Test Plan

Run jest tests.

Related PRs

#15175

Release Notes

[GENERAL][ENHANCEMENT][MINOR][View] - Add platform hook for adding View style props

[hramos]

You asked elsewhere how this might impact the docs. Right now, elevation is documented as part of the View Style Props docs. I've opened a PR on the docs repo with a proposal on how this might be documented: facebook/react-native-website#223

[facebook-github-bot]

@hramos is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[facebook-github-bot]

I tried to merge this pull request into the Facebook internal repo but some checks failed. To unblock yourself please check the following: Does this pull request pass all open source tests on GitHub? If not please fix those. Does the code still apply cleanly on top of GitHub master? If not can please rebase. In all other cases this means some internal test failed, for example a part of a fb app won't work with this pull request. I've added the Import Failed label to this pull request so it is easy for someone at fb to find the pull request and check what failed. If you don't see anyone comment in a few days feel free to comment mentioning one of the core contributors to the project so they get a notification.

[hramos]

It looks like with this change, some of our own internal apps are failing to build. I'll need to add this to my queue of diffs that need to be manually landed.

[rozele]

@hramos - any luck?

[hramos]

I haven't gotten around to it yet. Some of our apps are failing to build with this error:

Invariant Violation: "elevation" is not a valid style property.

[rozele]

@hramos - I imagine this may be because the elevation prop is set on iOS even if it's not used.

[rozele]

@hramos - perhaps we just leave elevation in the original ViewStylePropTypes file, any chance you could produce an isolated repro of the tests so I can try to come up with a workaround? It seem either:

1. We shouldn't produce errors when a platform-specific prop is set that isn't relevant to another platform.
2. We should provide a simple syntax recommendation for setting a prop on only the relevant platform, something like:

...Platform.select({
  ios: {},
  android: {
    elevation: 1
  },
  windows: {}
})

[cpojer]

We are currently working on getting rid of PropTypes altogether and using Flow Types to describe View props. I'm gonna close this PR and I'm sorry we didn't get to it earlier.