Call logger from within YGConfig

Summary:
@public

Stricter encapsulation of logging callbacks within `YGConfig`.

Instead of invoking the logging callback directly (`node->logger(...)`), callers now have to go through `YGConfig::log()`.

This change will allow us to add the concept of a *Layout Context,* where logging functions will be able to receive an additional `void *` argument if configured accordingly. This API will be used internally for Yoga’s JNI bindings, to avoid storing a weak JNI reference for each node, and avoid reference table overflows.

Changed API:

- `YGConfig::logger()` -> `YGConfig::log()`

Reviewed By: SidharthGuglani

Differential Revision: D14123483

fbshipit-source-id: 87b8bb7de0e4346b6a41e57a70ac4eb8d79b24af

yoga/log.cpp

@@ -24,7 +24,7 @@ void vlog(
     const char* format,
     va_list args) {
   YGConfig* logConfig = config != nullptr ? config : YGConfigGetDefault();
-  logConfig->logger(logConfig, node, level, format, args);
+  logConfig->log(logConfig, node, level, format, args);
 
   if (level == YGLogLevelFatal) {
     abort();