Create feature flag for DOM trees in Fabric

Summary:
This adds a feature flag to enable all the new DOM traversal and layout APIs (in react-native-community/discussions-and-proposals#607).

Changelog: [Internal]

Reviewed By: rshest

Differential Revision: D43981608

fbshipit-source-id: 77bb1ee4faaaf30cfc8bb2e493763b168702f498

Author: rubennorte

Diff for: Libraries/ReactNative/ReactNativeFeatureFlags.js

@@ -41,6 +41,10 @@ export type FeatureFlags = {|
    * implementation
    */
   isGlobalWebPerformanceLoggerEnabled: () => boolean,
+  /**
+   * Enables access to the host tree in Fabric using DOM-compatible APIs.
+   */
+  enableAccessToHostTreeInFabric: () => boolean,
 |};
 
 const ReactNativeFeatureFlags: FeatureFlags = {
@@ -50,6 +54,7 @@ const ReactNativeFeatureFlags: FeatureFlags = {
   animatedShouldDebounceQueueFlush: () => false,
   animatedShouldUseSingleOp: () => false,
   isGlobalWebPerformanceLoggerEnabled: () => false,
+  enableAccessToHostTreeInFabric: () => false,
 };
 
 module.exports = ReactNativeFeatureFlags;