out of tree platform sourcemap URL incorrectly lacks http protocol

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Report a bug

What is the current behavior?
See microsoft/react-native-windows#2482.

Sourcemap URLs end up with the format //localhost:8081/App.uwp.map?platform=uwp... on react-native-windows, which breaks because of the Blob loading done in the debugger UI.
Metro specifically fixes this on android and iOS here:

        // The remote chrome debugger loads bundles via Blob urls, whose
        // protocol is blob:http. This breaks loading source maps through
        // protocol-relative URLs, which is why we must force the HTTP protocol
        // when loading the bundle for either iOS or Android.
        protocol:
          platform != null && platform.match(/^(android|ios)$/) ? 'http' : '',

but because react-native-windows uses "uwp" as the platform, it continues to have the same issue.

If the current behavior is a bug, please provide the steps to reproduce and a minimal repository on GitHub that we can yarn install and yarn test.
See repro steps in microsoft/react-native-windows#2482

What is the expected behavior?
metro returns a sourceMapUrl with a protocol of http, instead of no protocol, on uwp platforms. Ideally Metro does not have to end up maintaining an exhaustive list of react native platforms, so perhaps a way to configure this behavior?

Please provide your exact Metro configuration and mention your Metro, node, yarn/npm version and operating system.
metro@0.49.2
node@v10.15.0
npm@6.5.0
OS: Windows 10

Start with Metro's packages/metro/src/lib/parseOptionsFromUrl.js, then follow the reproduction referenced in microsoft/react-native-windows#2482. Check how out-of-tree platforms produce sourceMapUrl values and verify that the uwp case returns an HTTP protocol without requiring an exhaustive platform list.