Actually define RCTSetLogThreshold()

Summary: This function was declared, but never defined, so calling it would crash your application.

I also took this opportunity to ensure that the logging threshold is given a default value upon initialization, not just the first time `RCTGetLogThreshold()` is called.

@​public

Reviewed By: @tadeuzagallo

Differential Revision: D2475622

Author: jspahrsummers

React/Base/RCTLog.m

@@ -32,21 +32,24 @@ - (void)logMessage:(NSString *)message level:(NSString *)level;
   "mustfix"
 };
 
+#if RCT_DEBUG
+static const RCTLogLevel RCTDefaultLogThreshold = RCTLogLevelInfo - 1;
+#else
+static const RCTLogLevel RCTDefaultLogThreshold = RCTLogLevelError;
+#endif
+
 static RCTLogFunction RCTCurrentLogFunction;
-static RCTLogLevel RCTCurrentLogThreshold;
+static RCTLogLevel RCTCurrentLogThreshold = RCTDefaultLogThreshold;
 
 RCTLogLevel RCTGetLogThreshold()
 {
-  if (!RCTCurrentLogThreshold) {
-#if RCT_DEBUG
-    RCTCurrentLogThreshold = RCTLogLevelInfo - 1;
-#else
-    RCTCurrentLogThreshold = RCTLogLevelError;
-#endif
-  }
   return RCTCurrentLogThreshold;
 }
 
+void RCTSetLogThreshold(RCTLogLevel threshold) {
+  RCTCurrentLogThreshold = threshold;
+}
+
 RCTLogFunction RCTDefaultLogFunction = ^(
   RCTLogLevel level,
   NSString *fileName,