RN: Add RootTag Type to TurboModule

Summary:
Adds `RootTag` as a valid type for arguments and return types in TurboModules (on both Android and iOS).

This will enable us to change `RootTag` into an opaque type. There are two compelling reasons to do this:

- JavaScript will no longer be able to safely depend on `RootTag` being a number (which means we can change this in the future).
- Call sites using `unstable_RootTagContext` will can get a `RootTag`, but call sites using the legacy `context.rootTag` will not. This means the opaque type will give us a strategy for migrating away from legacy context and eventually making `unstable_RootTagContext` the only way to access `RootTag`.

Changelog:
[Internal]

(Note: this ignores all push blocking failures!)

Reviewed By: RSNara

Differential Revision: D21127170

fbshipit-source-id: baec9d7ad17b2f8c4527f1a84f604fc0d28b97eb

RNTester/js/examples/TurboModule/SampleTurboModuleExample.js

@@ -11,6 +11,7 @@
 'use strict';
 
 import NativeSampleTurboModule from '../../../../Libraries/TurboModule/samples/NativeSampleTurboModule';
+import type {RootTag} from '../../../../Libraries/ReactNative/RootTag';
 import {
   StyleSheet,
   Text,
@@ -18,6 +19,7 @@ import {
   FlatList,
   Platform,
   TouchableOpacity,
+  unstable_RootTagContext,
 } from 'react-native';
 import * as React from 'react';
 
@@ -33,6 +35,8 @@ type State = {|
 |};
 
 class SampleTurboModuleExample extends React.Component<{||}, State> {
+  static contextType: React$Context<RootTag> = unstable_RootTagContext;
+
   state: State = {
     testResults: {},
   };
@@ -64,6 +68,7 @@ class SampleTurboModuleExample extends React.Component<{||}, State> {
       ]),
     getObject: () =>
       NativeSampleTurboModule.getObject({a: 1, b: 'foo', c: null}),
+    getRootTag: () => NativeSampleTurboModule.getRootTag(this.context),
     getValue: () =>
       NativeSampleTurboModule.getValue(5, 'test', {a: 1, b: 'foo'}),
   };