[RCTBridge] Have RCTBridge.loading return RCTBatchedBridge.loading

Summary:
The parent RCTBridge no longer tracks the JS loading since that has been handed off to the RCTBatchedBridge. To make the `loading` property accurate again, just expose the batch bridge's loading property from the parent bridge (note: I didn't make it KVO-compliant).

Fixes #1199
Closes #1200
Github Author: James Ide <ide@jameside.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

React/Base/RCTBridge.m

@@ -836,6 +836,11 @@ - (void)setUp
   _batchedBridge = [[RCTBatchedBridge alloc] initWithParentBridge:self];
 }
 
+- (BOOL)isLoading
+{
+  return _batchedBridge.loading;
+}
+
 - (BOOL)isValid
 {
   return _batchedBridge.isValid;