[JSContext] Define `ContextExecutor.setContextName` for debugging

Summary:
Add a method that lets JS set the name of the JSContext for debugging purposes. I check `JSGlobalContextSetName` since it is not available on iOS 7.

Closes https://github.com/facebook/react-native/pull/2144
Github Author: James Ide <ide@jameside.com>

React/Executors/RCTContextExecutor.m

@@ -557,4 +557,13 @@ static NSError *RCTNSErrorFromJSError(JSContextRef context, JSValueRef jsError)
   }), @"js_call,json_call", (@{@"objectName": objectName}))];
 }
 
+RCT_EXPORT_METHOD(setContextName:(NSString *)name)
+{
+  if (JSGlobalContextSetName != NULL) {
+    JSStringRef JSName = JSStringCreateWithCFString((__bridge CFStringRef)name);
+    JSGlobalContextSetName(_context.ctx, JSName);
+    JSStringRelease(JSName);
+  }
+}
+
 @end