Fix incorrect results from css_usesFlexbox

Summary: With the current implementation, as long as you had set a value on usesFlexbox (YES or NO), it would return YES. This fixes it.

Reviewed By: amonshiz

Differential Revision: D4124485

fbshipit-source-id: 32fe4ec0109b5c8678a112f9d4295bd17a6f6dc2

uikit/CSSLayout/Tests/CSSLayoutTests.m

@@ -46,4 +46,16 @@
   XCTAssertEqual(0, CSSNodeGetInstanceCount());
 }
 
+- (void)testUsesFlexbox
+{
+  UIView *view = [[UIView alloc] initWithFrame:CGRectZero];
+  XCTAssertFalse([view css_usesFlexbox]);
+
+  [view css_setUsesFlexbox:YES];
+  XCTAssertTrue([view css_usesFlexbox]);
+
+  [view css_setUsesFlexbox:NO];
+  XCTAssertFalse([view css_usesFlexbox]);
+}
+
 @end

uikit/CSSLayout/UIView+CSSLayout.m

@@ -56,11 +56,8 @@
 
 - (BOOL)css_usesFlexbox
 {
-  if (objc_getAssociatedObject(self, @selector(css_usesFlexbox))) {
+  NSNumber *usesFlexbox = objc_getAssociatedObject(self, @selector(css_usesFlexbox));
-    return YES;
+  return [usesFlexbox boolValue];
-  } else {
-    return NO;
-  }
 }
 
 - (void)css_setDirection:(CSSDirection)direction