Fabric: Unifying usage of autos

Summary:
I was watching a classic magnificent talk about modern C++ by Herb Sutter and I was totally sold on double down on using `auto` in our codebase. Surprisingly, 95% of the code base already follows Herb's guidence; I just changed the last 5% to make it consistent.
All those changes must work *exactly* like it was before.
The talk: https://youtu.be/xnqTKD8uD64?t=28m25s

Reviewed By: mdvacca

Differential Revision: D9753301

fbshipit-source-id: 9629aa485a5d6e51806cc96306c297284d4f90b8

ReactCommon/fabric/components/image/ImageShadowNode.cpp

@@ -59,7 +59,7 @@ ImageSource ImageShadowNode::getImageSource() const {
   auto targetImageArea = size.width * size.height * scale * scale;
   auto bestFit = kFloatMax;
 
-  ImageSource bestSource;
+  auto bestSource = ImageSource {};
 
   for (const auto &source : sources) {
     auto sourceSize = source.size;