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/YGConfig.cpp

@@ -6,4 +6,13 @@
  */
 #include "YGConfig.h"
 
-YGConfig::YGConfig(YGLogger logger) : logger(logger) {}
+YGConfig::YGConfig(YGLogger logger) : logger_{logger} {}
+
+void YGConfig::log(
+    YGConfig* config,
+    YGNode* node,
+    YGLogLevel logLevel,
+    const char* format,
+    va_list args) {
+  logger_(config, node, logLevel, format, args);
+}