Map accessibilityRole: grid to UIAccessibilityTraitNone

Summary:
D38121921 (5ddb997) added a grid accessibilityRole to Flow typings and RNTester example shared with iOS. It it forwarded on Android, but doesn't have an equivalent UIAccessibilityTrait mapping.`RNTesterSnapshotTests/testScrollViewExample` reports an error:
```
Failure: RedBox errors: (
    "Error setting property 'accessibilityRole' of RCTScrollView with tag #125: Invalid UIAccessibilityTraits 'grid'. should be one of: (
    adjustable,
====================== 38 lines skipped ======================
) (NSInternalInconsistencyException)
Path: <unknown>
Line: 0
```

This adds the grid mapping, which I think should fix this error.

Changelog:
[ios][Fixed] - Map `accessibilityRole: grid` to `UIAccessibilityTraitNone`

Reviewed By: christophpurrer

Differential Revision: D40848904

fbshipit-source-id: 80f72bcd4e4826cc0d535693117a6c1e5fbd1d7d

React/Views/RCTViewManager.m

@@ -64,6 +64,7 @@ @implementation RCTConvert (UIAccessibilityTraits)
       @"timer" : @(UIAccessibilityTraitNone),
       @"toolbar" : @(UIAccessibilityTraitNone),
       @"list" : @(UIAccessibilityTraitNone),
+      @"grid" : @(UIAccessibilityTraitNone),
     }),
     UIAccessibilityTraitNone,
     unsignedLongLongValue)