iOS: make RCTSurfaceHostingView have default backgroundColor

Summary:
Previously it defaults to using transparent color (iOS default), but when using `RCTSurfaceHostingProxyRootView` we actually manually set to `[UIColor whiteColor]`. However, if the surface is initialized via a different API, the color wasn't set. To avoid confusion and backward incompatibility, let's just set the same background color here.

We can decide in the future if the default color should be transparent instead.

Changelog: [Fixed][iOS] RCTSurfaceHostingView default background color is now consistent with RCTRootView

Reviewed By: RSNara

Differential Revision: D27973748

fbshipit-source-id: c506afbc5629df6647277aa2323f084773c8e760

React/Base/Surface/SurfaceHostingView/RCTSurfaceHostingProxyRootView.mm

@@ -74,7 +74,7 @@ - (instancetype)initWithBridge:(RCTBridge *)bridge
                                                        initialProperties:initialProperties];
   [surface start];
   if (self = [super initWithSurface:surface sizeMeasureMode:sizeMeasureMode]) {
-    self.backgroundColor = [UIColor whiteColor];
+    // Nothing specific to do.
   }
 
   RCT_PROFILE_END_EVENT(RCTProfileTagAlways, @"");

React/Base/Surface/SurfaceHostingView/RCTSurfaceHostingView.mm

@@ -60,6 +60,9 @@ - (instancetype)initWithSurface:(id<RCTSurfaceProtocol>)surface
     _surface.delegate = self;
     _stage = surface.stage;
     [self _updateViews];
+
+    // For backward compatibility with RCTRootView, set a color here instead of transparent (OS default).
+    self.backgroundColor = [UIColor whiteColor];
   }
 
   return self;