Include CSSLayout.h consistently with other project includes

Summary:
Include CSSLayout headers in the same way as other project headers, ie `#import <CSSLayout/CSSLayout.h>` becomes `#import "CSSLayout.h"`. CSSLayout is not a framework or system dependency, so shouldn't (AFAIK) be included with angle brackets. Doing so breaks framework builds, such as when RN is used as a pod in a swift project.

In combination with https://github.com/facebook/css-layout/pull/217 this fixes https://github.com/facebook/react-native/issues/9014 (specifically swift cocoapods projects). There is then no need for a separate CSSLayout pod subspec.

Tests run on the RN project in isolation (with changes inside `CSSLayout` itself also applied) and against a dummy swift project with RN included as a pod.

NB: This effectively reverts https://github.com/facebook/react-native/pull/9015 and may break non-swift cocoapods projects unless https://github.com/facebook/css-layout/pull/217 is merged and synced first.

Update: As discussed with alloy and emilsjolander, wrap these imports in a preprocess
Closes https://github.com/facebook/react-native/pull/9544

Differential Revision: D3821791

Pulled By: javache

fbshipit-source-id: d27ac8be9ce560d03479b43d3db740cd196c24da

React/Base/RCTConvert.h

@@ -10,7 +10,13 @@
 #import <QuartzCore/QuartzCore.h>
 #import <UIKit/UIKit.h>
 
+//Internally we reference a separate library. See https://github.com/facebook/react-native/pull/9544
+#if __has_include(<CSSLayout/CSSLayout.h>)
 #import <CSSLayout/CSSLayout.h>
+#else
+#import "CSSLayout.h"
+#endif
+
 #import "RCTAnimationType.h"
 #import "RCTBorderStyle.h"
 #import "RCTTextDecorationLineType.h"