[ReactNative] Fix timers for debugger executors

Summary:
@public

`-[RCTJavaScriptExecutor executeBlockOnJavaScriptQueue:]` would always `dispatch_async`
for the WebView and WebSocket executors, what caused for any frame aligned dispatch.

Test Plan:
Test the `Timers, TimerMixin` example on UIExplorer, `requestAnimationFrame` was
taking ~33.3ms when debugging, now takes ~16.6ms as expected.

Libraries/WebSocket/RCTWebSocketExecutor.m

@@ -185,6 +185,15 @@ RCT_EXPORT_MODULE()
 }
 
 - (void)executeBlockOnJavaScriptQueue:(dispatch_block_t)block
+{
+  if ([NSThread isMainThread]) {
+    block();
+  } else {
+    dispatch_async(dispatch_get_main_queue(), block);
+  }
+}
+
+- (void)executeAsyncBlockOnJavaScriptQueue:(dispatch_block_t)block
 {
   dispatch_async(dispatch_get_main_queue(), block);
 }