Add suggested ‘view size’ powers to -[RCTShadowView setFrame:]

Summary:The UICollectionView example is actually my use-case, which is discussed in a
bit more detail [here](https://github.com/alloy/ReactNativeExperiments/issues/2).

----

This is useful when wrapping native iOS components that determine their
own suggested size and which would be too hard/unnecessary to replicate
in the shadow view. For instance a `UICollectionView` that after layout
will update its `contentSize`, which could be used to suggest a size to
the shadow view.

The reason for adding it to -[RCTShadowView setFrame:] is mainly so it
can be used via the existing -[RCTUIManager setFrame:forView:] API and
because it might not be a feature you want to expose too prominently.

An origin of `{ NAN, NAN }` is used as a sentinel to indicate that the
frame should be used as a size suggestion. The size portion of the rect
may contain a `NAN` to skip that dimension or a suggested value for the
dimension which will be used if no explicit styling has been assigned.

Examples:

* Without any expl
Closes https://github.com/facebook/react-native/pull/6114

Differential Revision: D2994796

Pulled By: nicklockwood

fb-gh-sync-id: 6dd3dd86a352ca7d31a0da38bc38a2859ed0a410
shipit-source-id: 6dd3dd86a352ca7d31a0da38bc38a2859ed0a410

runXcodeTests.sh

@@ -10,16 +10,9 @@ if [ -z "$1" ]
     exit 255
 fi
 
-xctool \
-  -project IntegrationTests/IntegrationTests.xcodeproj \
-  -scheme IntegrationTests \
-  -sdk iphonesimulator8.1 \
-  -destination "platform=iOS Simulator,OS=${1},name=iPhone 5" \
-  build test
-
 xctool \
   -project Examples/UIExplorer/UIExplorer.xcodeproj \
   -scheme UIExplorer \
-  -sdk iphonesimulator8.1 \
+  -sdk iphonesimulator${1} \
   -destination "platform=iOS Simulator,OS=${1},name=iPhone 5" \
   build test